Node apparatus, optical wavelength division multiplexing network, and system switching method

ABSTRACT

An optical wavelength division multiplexing network which can carry out large-capacity access service with a simple constitution is provided. In an optical network comprising a plurality of layers, a highest level network comprises a ring network having a center node and remote nodes; an intermediate level network comprises a ring centered around a node belonging to the higher level network, and a lowest level network comprises a star network centered around an access node which multiplexes traffic from a plurality of ONU, the ONU and the access node being directly joined together by optical fibers; the center node belonging to the highest level network and the ONU establish a direct communication path by using lights of different wavelengths; at nodes provided therebetween, the optical signals are not electrically processed but are amplified, branched, and routed.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical wavelength divisionmultiplexing network which multiplexes and transmits optical signalshaving a plurality of different wavelengths.

[0003] 2. Description of the Related Art

[0004]FIG. 25 shows an example of the constitution of a conventionaloptical wavelength division multiplexing network. The network shown inFIG. 25 has a ring structure comprising two or three layers. The networkof FIG. 25 will be explained in separate sections comprising a network(1) 11, a network (2) 12, a network (3) 13, a network (4) 14, and anetwork (5) 15. The network (1) 11 has a ring structure, and is providedat the highest level. The network (1) 11 comprises at least one centernode 21 and two or more (three in FIG. 1) remote nodes 22, 23, and 24.The network (2) 12 is a ring network comprising a node (#4) 24, which isone of the remote nodes of the network (1) 11, and is provided below thenetwork (1) 11. The network (3) 13 comprises a tree-shaped structurecentered on a node (#41) 25, which is one of the nodes of the network(2) 12, and is provided below the network (2) 12. The network (4) 14comprises a ring-shaped structure centered on a node (#3) 23, which isone of the remote nodes of the network (1) 11, and is provided below thenetwork (1) 11. The network (5) 15 comprises a tree-shaped structurecentered on a node 27, which is one of a plurality of nodes 23, 26, 27,and 28 of the network (4) 14, and can conceivably be provided below thenetwork (4) 14. Optical network units (ONU) (they are also calledoptical service units.) 51 to 56 comprise the subscribers of each home,business office, and the like, and are provided in the network (3) 13 orthe network (5) 15.

[0005] In FIG. 25, the nodes of network (1) 11, the network (2) 12, andthe network (4) 14, are connected by optical fiber transmission paths60, 60, . . . which comprise a plurality of optical fibers. The ONUs areconnected to the nodes of the networks (3) 13 and (5) 15 by opticalfiber transmission paths 70, 70, . . . which comprise a single opticalfiber. Equipment for signal termination electrically processestransmission signals, which have been converted from optical signals toelectrical signals, and are provided at the nodes 21 to 28.

[0006] In this explanation, traffic from subscribers in the network (3)13 or the network (5) 15 is assumed to be 1.5 Mb/s. The traffic from thesubscribers is multiplexed at the subscriber office (node 25 or node27), and transmitted to the node (#4) 24 in the network (2) 12 or theremote node (#3) 23 in the network (1) 11 at a higher transmission speedof, for example, 52 Mb/s. At the node (#41) 25 and the node 27, trafficsent from other nodes in the network (2) 12 or the network (4) 14 iscombined with the multiplexed traffic from the subscribers, andtransmitted to the next node in the network (2) 12 or the network (4) 14at an even high transmission speed. Transmission speed conversion andthe like is also carried out at the nodes in the network (1) 11. Thatis, electrical processing is carried out at each node.

[0007] In conventional optical networks such as that shown in FIG. 25,when starting a new high-speed access service for users at a speed of,for example, approximately 150 Mb/s or 1 Gb/s, transmission apparatuseswhich carry out electrical processing for multiplexing the traffic mustbe provided at each node, since there are several users belonging to thenetwork (3) 13 or the network (5) 15. Consequently, the initialexpenditure is considerable. Moreover, depending on the region, theremay be fewer users per node, leading to a drawback of expenditureefficiency.

SUMMARY OF THE INVENTION

[0008] Accordingly, it is an object of this invention to provide anoptical wavelength division multiplexing network which can carry outlarge-capacity optical access services with a simpler constitution. Itis a more specific object of this invention to provide the opticalwavelength division multiplexing network which enables initialexpenditure to be reduced in large-capacity optical access servicesusing ONU.

[0009] In order to solve the problems mentioned above, a first aspect ofthis invention provides an optical wavelength division multiplexingnetwork having a structure comprising at least two layers, a highestlevel network being a ring network which comprises at least one centernode and two or more remote nodes which are joined by at least twooptical fibers; in the case where the layered structure comprises threeor more layers, excepting the lowest level network the intermediatelevel network comprising a ring having the node belonging to the highestlevel network as its center node, nodes belonging to the ring networkbeing joined by at least two optical fibers; the lowest level networkcomprising a star network centered around an access node whichmultiplexes traffic from one or a plurality of optical network units(ONU), the ONU and the access node being directly joined by at least oneoptical fiber; the remote nodes amplifying optical wavelength divisionmultiplexing signals which are transmitted on an optical fibercomprising the higher level network which the remote nodes belong to,branching the signals to an optical fiber comprising the lower levelnetwork, and coupling optical wavelength division multiplexing signals,input from an optical fiber comprising the lower level network, tooptical wavelength division multiplexing signals transmitted on anoptical fiber comprising the higher level network, and amplifying thecoupled signals; the access node amplifying the optical wavelengthdivision multiplexing signals transmitted from the optical fibers whichcomprise the higher level network which the access node is connected to,selecting optical signals having wavelengths which correspond to theONU, and outputting the selected signals to the ONU; multiplexing theoptical signals transmitted from the ONU, dividing the multiplexedsignals in a plurality of directions, amplifying the divided signals,and transmitting the amplified signals to an optical fiber comprising ahigher level network which the access node is connected to; and thecenter node belonging to the highest level network and the ONUestablishing a direct communication path by using lights of differentwavelengths, the optical signals being amplified, branched, and routedat the remote nodes and the access node provided therebetween.

[0010] Furthermore, a first aspect of the center node comprising theoptical wavelength division multiplexing network according to the firstaspect described above comprising: a plurality of opticalde-multiplexers which de-multiplex optical wavelength divisionmultiplexing signals, input from optical fibers comprising the highestlevel network, to optical signals at each wavelength; a plurality ofoptical receivers which convert the optical signals which have beende-multiplexed by the optical de-multiplexers to electrical signals; aplurality of selectors which selectively output either of the outputsfrom the plurality of optical receivers; a signal termination sectionwhich performs predetermined electrical processing to the electricalsignals which have been selected by the selectors, and outputs aplurality of groups of electrical signals; a plurality of opticalsenders which convert the electrical signals output from the signaltermination section to a plurality of optical signals having differentwavelengths; and a plurality of optical multiplexers which multiplex theoptical signals output from the optical senders, and output themultiplexed signals to optical fibers comprising the highest levelnetwork.

[0011] A second aspect of the center node comprising the opticalwavelength division multiplexing network according to the first aspectdescribed above comprising: a plurality of optical de-multiplexers whichde-multiplex optical wavelength division multiplexing signals, inputfrom optical fibers comprising the highest level network, to opticalsignals at each wavelength; a plurality of optical switches which selectone of the optical signal which have been de-multiplexed by the opticalde-multiplexers; a plurality of optical receivers which convert theoptical signals which have been selected by the optical switches toelectrical signals; a signal termination section which performspredetermined electrical processing to the electrical signals which havebeen output from the optical receivers, and outputs a plurality ofgroups of electrical signals; a plurality of optical senders whichconvert the electrical signals output from the signal terminationsection to a plurality of optical signals having different wavelengths;and a plurality of optical multiplexers which multiplex the opticalsignals output from the optical senders, and output the multiplexedsignals to optical fibers comprising the highest level network.

[0012] A third aspect of the center node comprising the opticalwavelength division multiplexing network according to the first aspectdescribed above comprising: a plurality of optical de-multiplexers whichde-multiplex optical wavelength division multiplexing signals, inputfrom optical fibers comprising the highest level network, to a pluralityof optical signals at each wavelength; a plurality of optical switcheswhich select one of the plurality of optical signals which have beende-multiplexed by the optical de-multiplexers; a plurality of opticalreceivers which convert the optical signals which have been selected bythe optical switches to electrical signals; a signal termination sectionwhich performs predetermined electrical processing to the electricalsignals which have been output from the optical receivers, and outputs aplurality of groups of electrical signals; a plurality of opticalsenders which convert the plurality of electrical signals output fromthe signal termination section to a plurality of optical signals havingdifferent wavelengths; a plurality of optical dividers which divide theoptical signals output from the optical senders in a plurality ofdirections; and a plurality of optical multiplexers which multiplex theplurality of optical signals output from the optical dividers, andoutput the multiplexed signals to optical fibers comprising the highestlevel network.

[0013] A remote node comprising the optical wavelength divisionmultiplexing network according to the first aspect described abovecomprising: passive optical components which branch optical signalstransmitted on an optical fiber comprising a higher level network to anoptical fiber comprising a lower level network, and couple opticalsignals input from an optical fiber comprising the lower level networkto optical signals transmitted on an optical fiber comprising the higherlevel network; and optical amplifiers which amplify the optical signalsinput to the passive optical components and the optical signals outputfrom the passive optical components.

[0014] An access node comprising the optical wavelength divisionmultiplexing network according to the first aspect described abovecomprising: an optical switch which selects one of the optical signalswhich are input from optical fibers comprising a higher level network; afirst optical amplifier which amplifies, among the optical signals whichare input from the optical fibers comprising the higher level network,at least the optical signal selected by the optical switch; an opticalmultiplexer/de-multiplexer which, based on the optical signal selectedby the optical switch, selects an optical signal having a wavelengthwhich corresponds to the ONU, outputs the selected signal to the ONU,and multiplexes the optical signals transmitted from the ONU; an opticaldivider which divides the optical signal, multiplexed by the opticalmultiplexer/de-multiplexer, into a plurality of directions, andtransmits the divided signals to the optical fibers comprising thehigher level network; and a second optical amplifier which amplifies theoptical signals which are transmitted to the optical fibers comprisingthe higher level network.

[0015] A second aspect of this invention provides an optical wavelengthdivision multiplexing network having a structure comprising at least twolayers, a highest level network being a ring network which comprises atleast one center node and two or more remote nodes which are joined byat least two optical fibers; a lowest level network comprising a starnetwork centered around an access node which multiplexes traffic fromone or a plurality of optical network units (ONU), the ONU and theaccess node being directly joined by at least one optical fiber; animmediately higher level network of the lowest level network being aring network comprising at least one the access node connected by atleast two fibers, traffic from the access nodes being multiplexed at acenter node in the ring network which the access node belongs to, andconnected by the center node to an even higher level network; the remotenode amplifying and branching optical wavelength division multiplexingsignals which are transmitted on an optical fiber comprising the higherlevel network which the remote node belongs to, de-multiplexing andreceiving only optical signals at wavelengths corresponding to the ONU,electrically processing the optical signals, and transmitting theprocessed signals at a predetermined wavelength to optical fiberscomprising a lower level network; de-multiplexing and receiving onlyoptical signals among the optical wavelength division multiplexingsignals, input along the optical fibers comprising the lower levelnetwork, which are at wavelengths corresponding to the ONU, electricallyprocessing the optical signals, converting the processed signals tooptical signals at wavelengths which were allocated beforehand, andcoupling the converted signals to optical wavelength divisionmultiplexing signals transmitted on optical fibers comprising the higherlevel network; the access node provided between the remote node and theONU amplifying the optical wavelength division multiplexing signalswhich are transmitted on the optical fibers comprising the higher levelnetwork which the access node is connected to, selecting optical signalswhich correspond to the ONU and outputting the selected signals thereto;and multiplexing the optical signals from the ONU, dividing themultiplexed signal in a plurality of directions, amplifying the dividedsignals, and transmitting the amplified signals on optical fiberscomprising the higher level network which the access node is connectedto; and optical signals having different wavelengths being transmittedbetween the ONU and the remote node in the higher level network, whichis the center node in the ring network comprising the access node, theaccess node provided between the remote node and the ONU amplifying androuting the optical signals.

[0016] A first aspect of the center node comprising the opticalwavelength division multiplexing network according to the second aspectdescribed above comprising: a plurality of optical de-multiplexers whichde-multiplex optical wavelength division multiplexing signals, inputfrom optical fibers comprising the highest level network, to opticalsignals at each wavelength; a plurality of optical receivers whichconvert the optical signals which have been de-multiplexed by theoptical de-multiplexers to electrical signals; a plurality of selectorswhich selectively output either of the outputs from the plurality ofoptical receivers; a signal termination section which performspredetermined electrical processing to the electrical signals which havebeen selected by the selectors, and outputs a plurality of groups ofelectrical signals; a plurality of optical senders which convert theelectrical signals output from the signal termination section to aplurality of optical signals having different wavelengths; and aplurality of optical multiplexers which multiplex the optical signalsoutput from the optical senders, and output the multiplexed signals tooptical fibers comprising the highest level network.

[0017] A second aspect of the center node comprising the opticalwavelength division multiplexing network according to the second aspectdescribed above comprising: a plurality of optical de-multiplexers whichde-multiplex optical wavelength division multiplexing signals, inputfrom optical fibers comprising the highest level network, to opticalsignals at each wavelength; a plurality of optical switches which selectone of the optical signal which have been de-multiplexed by the opticalde-multiplexers; a plurality of optical receivers which convert theoptical signals which have been selected by the optical switches toelectrical signals; a signal termination section which performspredetermined electrical processing to the electrical signals which havebeen output from the optical receivers, and outputs a plurality ofgroups of electrical signals; a plurality of optical senders whichconvert the electrical signals output from the signal terminationsection to a plurality of optical signals having different wavelengths;and a plurality of optical multiplexers which multiplex the opticalsignals output from the optical senders, and output the multiplexedsignals to optical fibers comprising the highest level network.

[0018] A third aspect of the center node comprising the opticalwavelength division multiplexing network according to the second aspectdescribed above comprising: a plurality of optical de-multiplexers whichde-multiplex optical wavelength division multiplexing signals, inputfrom optical fibers comprising the highest level network, to a pluralityof optical signals at each wavelength; a plurality of optical switcheswhich select one of the plurality of optical signals which have beende-multiplexed by the optical de-multiplexers; a plurality of opticalreceivers which convert the optical signals which have been selected bythe optical switches to electrical signals; a signal termination sectionwhich performs predetermined electrical processing to the electricalsignals which have been output from the optical receivers, and outputs aplurality of groups of electrical signals; a plurality of opticalsenders which convert the plurality of electrical signals output fromthe signal termination section to a plurality of optical signals havingdifferent wavelengths; a plurality of optical dividers which divide theoptical signals output from the optical senders in a plurality ofdirections; and a plurality of optical multiplexers which multiplex theplurality of optical signals output from the optical dividers, andoutput the multiplexed signals to optical fibers comprising the highestlevel network.

[0019] A remote node comprising the optical wavelength divisionmultiplexing network according to the second aspect described abovecomprising: passive optical components which branch optical signalstransmitted on optical fibers comprising the higher level network, andcouple input optical signals to optical signals transmitted on opticalfibers comprising the higher level network; optical amplifiers whichamplify the optical signals input to the passive optical components andthe optical signals output from the passive optical components; and anequipment for signal termination which de-multiplexes only the opticalsignals among those divided by the passive optical components atwavelengths corresponding to the ONU, receives and electricallyprocesses the optical signals at each wavelength, and transmits theprocessed signals at a predetermined wavelength, and in addition,de-multiplexes only the optical signals among those input along theoptical fibers comprising a lower level network which are at wavelengthscorresponding to the ONU, receives and electrically processes theoptical signals at each wavelength, converts the processed signals tooptical signals at a wavelength allocated beforehand, and transmits theconverted signals to the passive optical components.

[0020] An access node comprising the optical wavelength divisionmultiplexing network according to the second aspect described abovecomprising: an optical switch which selects one of the optical signalswhich are input from optical fibers comprising a higher level network; afirst optical amplifier which amplifies, among the optical signals whichare input from the optical fibers comprising the higher level network,at least the optical signal selected by the optical switch; an opticalmultiplexer/de-multiplexer which, based on the optical signal selectedby the optical switch, selects an optical signal having a wavelengthwhich corresponds to the ONU, outputs the selected signal to the ONU,and multiplexes the optical signals transmitted from the ONU; an opticaldivider which divides the optical signal, multiplexed by the opticalmultiplexer/de-multiplexer, into a plurality of directions, andtransmits the divided signals to the optical fibers comprising thehigher level network; and a second optical amplifier which amplifies theoptical signals which are transmitted to the optical fibers comprisingthe higher level network.

[0021] A third aspect of this invention provides an optical wavelengthdivision multiplexing network having a structure comprising at leastthree layers, a highest level network being a ring network comprising atleast one center node and two or more remote nodes which are joined byat least four optical fibers; an intermediate level network being a ringnetwork having a node belonging to the higher level network as a centernode thereof, access nodes belonging to the ring network being joined byat least four optical fibers; a lowest level network comprising a starnetwork centered around an access node which multiplexes traffic fromoptical network units (ONU), the ONU and the access node being directlyjoined by at least one optical fiber; the remote node amplifying opticalwavelength division multiplexing signals transmitted on the opticalfibers comprising a higher level node which the remote node belongs to,branching the signals to optical fibers comprising a lower levelnetwork, and coupling optical wavelength division multiplexing signalswhich are input from optical fibers comprising the lower level networkto optical wavelength division multiplexing signals transmitted onoptical fibers comprising the higher level network, thereby amplifyingthe coupled signals; the access node amplifying optical wavelengthdivision multiplexing signals transmitted on optical fibers comprising ahigher level network, which the access node belongs to, branching theamplified signals to a lower level network for outputting the branchedsignals to the ONU; multiplexing optical signals transmitted from theONU, dividing the multiplexed signals in a plurality of directions,coupling the divided signal to optical wavelength division multiplexingsignals transmitted on optical fibers comprising a higher level networkwhich the access node is connected to, and amplifying the coupledsignals; and the center node belonging to the highest level network andthe ONU establishing a direct communication path by using lights ofdifferent wavelengths, the optical signals being amplified, branched, orrouted, at the remote nodes and the access nodes provided therebetween.

[0022] A fourth aspect of this invention provides an optical wavelengthdivision multiplexing network having a structure comprising at leastthree layers, a highest level network being a ring network comprising atleast one center node and two or more remote nodes which are joined byat least two optical fibers; an intermediate level network being a ringnetwork having a node belonging to the higher level network as a centernode thereof, access nodes belonging to the ring network being joined byat least four optical fibers; a lowest level network comprising a starnetwork centered around an access node which multiplexes traffic fromoptical network units (ONU), the ONU and the access node being directlyjoined by at least one optical fiber; the remote nodes amplifyingoptical wavelength division multiplexing signals transmitted on theoptical fibers comprising a higher level network which the remote nodesbelong to, branching the signals to optical fibers comprising a lowerlevel network, and coupling optical wavelength division multiplexingsignals which are input from optical fibers comprising the lower levelnetwork to optical wavelength division multiplexing signals transmittedon optical fibers comprising the higher level network, and amplifyingthe coupled signals; the access node amplifying optical wavelengthdivision multiplexing signals transmitted on optical fibers comprising ahigher level network, which the access node belongs to, branching themto a lower level network for outputting the branched signals to the ONU;multiplexing optical signals transmitted from the ONU, dividing them ina plurality of directions, coupling the divided signals to opticalwavelength division multiplexing signals transmitted on optical fiberscomprising a higher level network which the access node is connected to,and amplifying the coupled signals; and the center node belonging to thehighest level network and the ONU establishing a direct communicationpath by using lights of different wavelengths, the optical signals beingonly amplified, branched, or routed, at the remote nodes and the accessnode provided therebetween.

[0023] A center node comprising the optical wavelength divisionmultiplexing network according to the third and fourth aspects describedabove comprising: a plurality of optical de-multiplexers whichde-multiplex optical wavelength division multiplexing signals, inputfrom optical fibers comprising the highest level network, to opticalsignals at each wavelength; a plurality of optical receivers whichconvert the optical signals which have been de-multiplexed by theoptical de-multiplexers to electrical signals; a plurality of selectorswhich selectively output either of the outputs from the plurality ofoptical receivers; a signal termination section which performspredetermined electrical processing to the electrical signals which havebeen selected by the selectors, and outputs a plurality of groups ofelectrical signals; a plurality of optical senders which convert theelectrical signals output from the signal termination section to aplurality of optical signals having different wavelengths; and aplurality of optical multiplexers which multiplex the optical signalsoutput from the optical senders, and output the multiplexed signals tooptical fibers comprising the highest level network.

[0024] A remote node comprising the optical wavelength divisionmultiplexing network according to the third aspect described abovecomprising: passive optical components which branch optical signalstransmitted on optical fibers comprising a higher level network tooptical fibers comprising a lower level network, and in addition, coupleoptical signals input from optical fibers comprising the lower levelnetwork to optical signals transmitted on optical fibers comprising thehigher level network; and optical amplifiers which amplify opticalsignals which are input to, and output from, the passive opticalcomponents; wherein both ends of the loop of optical fibers comprisingthe lower level network are opened by using optical terminators.

[0025] An access node comprising the optical wavelength divisionmultiplexing network according to the third and fourth aspects describedabove comprising: first passive optical components which branch opticalsignals transmitted on optical fibers comprising a higher level networkto a lower level network; an optical switch which selects one of theoptical signals which have been branched by the first passive opticalcomponents; an optical multiplexer/de-multiplexer which transmits theoptical signals selected by the optical switch toward the ONU, andmultiplexes the optical signals transmitted from the ONU; an opticaldivider which divides the optical signals multiplexed by the opticalmultiplexer/de-multiplexer in a plurality of directions; second passiveoptical components which couple optical signals divided by the opticaldivider to optical signals transmitted on optical fibers comprising thehigher level network; and optical amplifiers which amplify the opticalsignals which are input to and output from the first and second passiveoptical components.

[0026] A remote node comprising the optical wavelength divisionmultiplexing network according to the fourth aspect described abovecomprising: passive optical components which branch optical signalstransmitted on optical fibers comprising a higher level network tooptical fibers comprising a lower level network, and in addition, coupleoptical signals input from optical fibers comprising the lower levelnetwork to optical signals transmitted on optical fibers comprising thehigher level network; and optical amplifiers which amplify opticalsignals transmitted on the optical fibers comprising the higher levelnetwork; wherein one end of the loop of optical fibers comprising thelower level network is opened by using optical terminators.

[0027] A fifth aspect of this invention provides an optical wavelengthdivision multiplexing network having a structure comprising at least twolayers, a highest level network comprising a ring network having atleast one center node and two or more remote nodes, which are joined byat least four optical fibers; intermediate level networks excepting thelowest level network comprising a ring network having a node belongingto the higher level network as a center node, and at least one nodebelonging to the intermediate level ring networks being joined by atleast four optical fibers; the lowest level network comprising a starnetwork centered around an access node belonging to the ring networkwhich is provided immediately thereabove, the access node being joinedto at least one optical network unit (ONU) by at least two opticalfibers; the remote nodes amplifying optical wavelength divisionmultiplexing signals transmitted on the optical fibers comprising ahigher level node which the remote nodes belong to, branching thesignals to optical fibers comprising a lower level network; and couplingoptical wavelength division multiplexing signals which are input fromoptical fibers comprising the lower level network to optical wavelengthdivision multiplexing signals transmitted on optical fibers comprisingthe higher level network; the access node amplifying optical wavelengthdivision multiplexing signals transmitted on optical fibers comprising ahigher level network which the access node is connected to, branchingthe amplified signals to a lower level network, amplifying the dividedsignals, and outputting the amplified signals to the ONU; multiplexingand amplifying optical signals transmitted from the ONU, dividing theamplified signals in a plurality of directions, coupling the dividedsignals to optical wavelength division multiplexing signals transmittedon optical fibers comprising a higher level network which the accessnode is connected to, and amplifying the coupled signals; and the centernode belonging to the highest level network transmitting data by usingdifferent wavelengths allocated to the ONU, the ONU transmitting thedata to the center node by using optical signals having the samewavelengths as the allocated wavelengths; and the access nodes and theremote nodes provided between the center node and the ONU onlyamplifying and dividing, or routing, the optical signals.

[0028] A center node comprising the optical wavelength divisionmultiplexing network according to the fifth aspect described abovecomprising: a plurality of optical de-multiplexers which de-multiplexoptical wavelength division multiplexing signals, input from opticalfibers comprising the highest level network, to optical signals at eachwavelength; a plurality of optical receivers which convert the opticalsignals which have been de-multiplexed by the optical de-multiplexers toelectrical signals; a plurality of selectors which selectively outputeither of the outputs from the plurality of optical receivers; a signaltermination section which performs predetermined electrical processingto the electrical signals which have been selected by the selectors, andoutputs a plurality of groups of electrical signals; a plurality ofoptical senders which convert the electrical signals output from thesignal termination section to a plurality of optical signals havingdifferent wavelengths; and a plurality of optical multiplexers whichmultiplex the optical signals output from the optical senders, andoutput the multiplexed signals to optical fibers comprising the highestlevel network.

[0029] A remote node comprising the optical wavelength divisionmultiplexing network according to the fifth aspect described abovecomprising: first passive optical components which branch opticalsignals transmitted on optical fibers comprising a higher level networkto optical fibers comprising a lower level network; second passiveoptical components which couple optical signals input from opticalfibers comprising the lower level network to optical signals transmittedon optical fibers comprising the higher level network; and opticalamplifiers which amplify optical signals which are input to, and outputfrom, the first and second passive optical components; wherein both endsof the loop of optical fibers comprising the lower level network areopened by using optical terminators.

[0030] An access node comprising the optical wavelength divisionmultiplexing network according to the fifth aspect described abovecomprising: first passive optical components which branch opticalsignals transmitted on optical fibers comprising a higher level networkto a lower level network; an optical switch which selects one of theoptical signals which have been branched by the first passive opticalcomponents; a first optical amplifier which amplifies, among the opticalsignals which have been branched by the first passive opticalcomponents, at least the optical signal selected by the optical switch;a second passive optical component which distributes the optical signalsamplified by the first optical amplifier to the ONU, and multiplexes theoptical signals transmitted from the ONU; a second optical amplifierwhich amplifies the optical signals multiplexed by the second passiveoptical component; an optical divider which divides the optical signal,amplified by the second optical amplifier, into a plurality ofdirections; a third passive optical component which couples the opticalsignals branched by the optical divider to an optical signal transmittedon optical fibers comprising the higher level network; and a thirdoptical amplifier which amplifies the optical signals which aretransmitted on the optical fibers comprising the higher level network.

[0031] In this invention, the center node belonging to the highest levelnetwork and the ONU establish a direct communication path by usinglights of different wavelengths. At the nodes therebetween, the signalsare amplified, branched, and routed in their optical format withoutbeing electrically processed. In other words, the center node of thenetwork which is the final multiplexing destination of the traffic canbe directly linked to the user by an optical signal at a certainwavelength. No electrical processing is performed at the nodes inbetween. The users and the center node are directly joined by opticalsignals at different wavelengths. In this case, only the center nodemultiplexes traffic from users, and carries out electrical processingsuch as communicating with other users in the regional network,distributing traffic to the core network, and the like. Therefore,according to this invention, it is possible to provide an opticalwavelength division multiplexing network which can carry outlarge-capacity access services with a simpler constitution.

[0032] Furthermore, according to a system switching method in theoptical wavelength division multiplexing network of this invention, whenan optical fiber (working fiber), which is being used in transmitting adown signal from the center node to the ONU in the higher level network,becomes severed, an access node belonging to a remote node provideddownstream than the severance point as seen from the center node,switches from the working fiber side to an optical fiber side(protection fiber) which is not presently in use, the down signal beingreceived after transmission along the protection fiber; when a workingfiber for transmitting an up signal from the ONU to the center node inthe higher level network has become severed, for an access nodebelonging to remote node where the severance point on the working fiberto the center node is located, the center node switches from the workingfiber to a protection fiber, and receives the up signal from theprotection fiber; and when an optical cable in the intermediate levelnetwork has become severed, an access node, among the access nodesconnected to the intermediate level network, which is provideddownstream than the severance point for the optical signal transmittedon the severed fiber switches from the working fiber to the protectionfiber and thereby receives the down signal; and at the access nodeprovided downstream, the center node switches from the working fiber tothe protection fiber and thereby receives the up signal from theprotection fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033]FIG. 1 is a block diagram showing the entire constitution of theoptical wavelength division multiplexing network in the embodiments ofthis invention;

[0034]FIG. 2 is a block diagram showing the constitution of a firstembodiment of this invention;

[0035]FIG. 3 is a block diagram showing the constitution of a secondembodiment of this invention;

[0036]FIG. 4 is a block diagram showing one example of the constitutionof a center node 21 a shown in FIGS. 1 to 3 and FIGS. 7 to 11;

[0037]FIG. 5 is a block diagram showing another example of theconstitution of the center node 21 a shown in FIGS. 1 to 3 and FIGS. 7to 11;

[0038]FIG. 6 is a block diagram showing yet another example of theconstitution of the center node 21 a shown in FIGS. 1 to 3 and FIGS. 7to 11;

[0039]FIG. 7 is a block diagram showing the constitution of a thirdembodiment of this invention;

[0040]FIG. 8 is a block diagram showing the constitution of a fourthembodiment of this invention;

[0041]FIG. 9 is a block diagram showing the constitution of a fifthembodiment of this invention;

[0042]FIG. 10 is a block diagram showing the constitution of a sixthembodiment of this invention;

[0043]FIG. 11 is a block diagram showing the constitution of a seventhembodiment of this invention;

[0044]FIG. 12 is a block diagram showing the constitution of an eighthembodiment of this invention;

[0045]FIG. 13 is a block diagram showing an opticalmultiplexer/de-multiplexer 115 shown in these diagrams when it isarranged as an AWG;

[0046]FIG. 14 is a diagram showing one example of the relationshipbetween the wavelength of the AWG 115 of FIG. 13 and the input/outputports;

[0047]FIG. 15 is a block diagram showing the constitution of a ninthembodiment of this invention;

[0048]FIG. 16 is a block diagram showing the constitution of the ninthembodiment of this invention;

[0049]FIG. 17 is a block diagram showing the constitution of the ninthembodiment of this invention;

[0050]FIG. 18 is a block diagram showing the constitution of the centernode 21 b of FIGS. 15 to 17;

[0051]FIG. 19 is a block diagram showing the constitution of a tenthembodiment of this invention;

[0052]FIG. 20 is a block diagram showing the constitution of the tenthembodiment of this invention;

[0053]FIG. 21 is a block diagram showing the constitution of the centernode 21 c of FIGS. 19 to 20;

[0054]FIG. 22 is a block diagram showing the constitution of an eleventhembodiment of this invention;

[0055]FIG. 23 is a block diagram showing the constitution of the centernode 21 d of FIG. 22;

[0056]FIG. 24 is a block diagram showing the constitution of a twelfthembodiment of this invention;

[0057]FIG. 25 is a block diagram showing one example of the constitutionof a conventional optical wavelength division multiplexing network;

[0058]FIG. 26 is a block diagram showing the constitution of athirteenth embodiment of this invention;

[0059]FIG. 27 is a block diagram showing the constitution of afourteenth embodiment of this invention; and

[0060]FIG. 28 is a block diagram showing the constitution of a fifteenthembodiment of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0061] Preferred embodiments of the optical wavelength divisionmultiplexing network according to this invention will be explained withreference to the drawings.

Embodiment 1

[0062]FIG. 1 is a block diagram showing the entire constitution of theoptical wavelength division multiplexing network in this embodiment andin embodiments subsequently explained. As shown in FIG. 1, parts of theconstitution which are identical to those in FIG. 25 are represented bythe same reference numerals. Parts of the constitution which correspondto those in FIG. 25 are specified by adding the letter “a” to the end ofthe reference numerals shown in FIG. 25.

[0063] The first embodiment will be explained with reference to FIGS. 1and 2. FIG. 2 is a block diagram showing the constitutions of thenetwork (1) 11 a, the network (2) 12 a, and the network (3) 13 a, shownin FIG. 1. This embodiment uses a two-fiber bi-directional ringconstitution, in which the nodes of the networks (1) 11 a, (2) 12 a,etc., are connected by pairs of optical fibers which transmit opticalsignals in differing directions. For example, in the network (1) 11 a,the nodes are connected by optical fibers (1) 62 and (2) 61, and opticalfibers (1) 64 and (2) 63, which transmit optical signals in differingdirections. In the network (2) 12 a, the nodes are connected by opticalfibers 65 and 66, and optical fibers 67 and 68, which transmit opticalsignals in differing directions.

[0064] In the example shown in FIG. 2, ONU 51, 52, and 53 are joined byoptical signal transmitters comprising the linear optical fibers 65, 66,67, 68, and the like, which extend from the node 24 a belonging to thenetwork (1) 11 a having the center node 21 a. In FIG. 2, the curvedsolid lines represent the fibers which are being used as working fibers(optical fibers 61, 62, 67, 68, etc.), and the dotted lines representthe fibers which are being used as protection fibers (optical fibers 63,64, 65, 66, etc.). The optical networks in this embodiment, and in theembodiments described subsequently, are characterized in that, as shownin FIG. 1, the optical signals are not electrically processed (i.e.processing performed when multiplexing traffic, such as converting thetransmission speed) at the remote nodes and access nodes (offices,telephone stations) other than the center node (21 a) belonging to thering network (network (1) 11 a) at the highest level.

[0065] This feature will be explained in detail. The constitution of thecenter node 21 a shown in FIGS. 1 and 2 will be explained with referenceto FIGS. 4 to 6. In FIGS. 4 to 6, signal lines for transmitting opticalsignal are represented by thick lines, and signal lines for transmittingelectrical signals are represented by fine lines.

[0066] In the constitution shown by way of example in FIG. 4, the centernode 21 a of FIG. 2 comprises an optical de-multiplexer 201 whichde-multiplexes a wavelength division multiplexing signal, which is inputfrom the remote node 22 a via the optical fiber (2) 61 and has ndifferent wavelengths from λn+1 to λ2n, to n optical signals at eachwavelength, an optical de-multiplexer 202 which de-multiplexes awavelength division multiplexing signal, which is input from the remotenode 24 a via the optical fiber (1) 64 and has n different wavelengthsfrom λn+1 to λ2n, to n optical signals at each wavelength, n opticalreceivers 221_n+1 to 221 _(—)2n and optical receivers 222_n+1 to 222_(—)2n which convert the n optical signals de-multiplexed by the opticalde-multiplexers 201 and 202 to electrical signals, n selectors 231_1 to231_n which selectively output either one of the outputs from theoptical receivers 221_n+1 to 221 _(—)2n and the optical receivers222_n+1 to 222 _(—)2n, an equipment for signal termination 241 whichperforms predetermined electrical processing to the electrical signalsoutput from the selectors 231_1 to 231_n and outputs two groups of nelectrical signals, n optical senders 251_1 to 251_n and optical senders252_1 to 252_n which convert the electrical signals output from theequipment for signal termination 241 to optical signals having ndifferent wavelengths from λ1 to λn, an optical multiplexer 211 whichmultiplexes the optical signals output from the optical senders 251_1 to251_n and outputs them to the optical fiber (1) 62, and an opticalmultiplexer 212 which multiplexes the optical signals output from theoptical senders 252_1 to 252_n and outputs them to the optical fiber (2)63.

[0067] In the constitution shown by way of example in FIG. 5, the centernode 21 a of FIG. 2 comprises an optical de-multiplexer 201, identicalto that in the constitution shown in FIG. 4, which de-multiplexes awavelength division multiplexing signal, which is input from the remotenode 22 a via the optical fiber (2) 61 and has n different wavelengthsfrom λn+1 to λ2n, to n optical signals at each wavelength, an opticalde-multiplexer 202 which de-multiplexes a wavelength divisionmultiplexing signal, which is input from the remote node 24 a via theoptical fiber (1) 64 and has n different wavelengths from λn+1 to λ2n,to n optical signals at each wavelength, n optical switches 261_n+1 to261 _(—)2n which selectively output either of the n optical signalsoutput from the optical de-multiplexers 201 and 202, n optical receivers271 _(—)1n+1 to 271 _(—)2n which convert the n optical signals outputfrom the optical switches 261_n+1 to 261 _(—)2n to electrical signals,an equipment for signal termination 241 which performs predeterminedelectrical processing to the electrical signals output from the opticalreceivers 271_n+1 to 271 _(—)2n and outputs two groups of n electricalsignals, n optical senders 251_1 to 251_n and optical senders 252_1 to252_n which convert the electrical signals output from the equipment forsignal termination 241 to optical signals having n different wavelengthsfrom λ1 to λn, an optical multiplexer 211 which multiplexes the opticalsignals output from the optical senders 251_1 to 251_n and outputs themto the optical fiber (1) 62, and an optical multiplexer 212 whichmultiplexes the optical signals output from the optical senders 252_1 to252_n and outputs them to the optical fiber (2) 63.

[0068] In the constitution shown by way of example in FIG. 6, the centernode 21 a of FIG. 2 comprises an optical de-multiplexer 201, identicalto that in the constitution shown in FIG. 5, which de-multiplexes awavelength division multiplexing signal, which is input from the remotenode 22 a via the optical fiber (2) 61 and has n different wavelengthsfrom λn+1 to λ2n, to n optical signals at each wavelength, an opticalde-multiplexer 202 which de-multiplexes a wavelength divisionmultiplexing signal, which is input from the remote node 24 a via theoptical fiber (1) 64 and has n different wavelengths from λn+1 to λ2n,to n optical signals at each wavelength, n optical switches 261_n+1 to261 _(—)2n which selectively output either of the n optical signalsoutput from the optical de-multiplexers 201 and 202, n optical receivers271_n+1 to 271 _(—)2n which convert the n optical signals output fromthe optical switches 261_n+1 to 261 _(—)2n to electrical signals, anequipment for signal termination 241 a which performs predeterminedelectrical processing to the electrical signals output from the opticalreceivers 271_n+1 to 271 _(—)2n and outputs one group of n electricalsignals, n optical senders 281_1 to 281_n which convert the electricalsignals output from the equipment for signal termination 241 a tooptical signals having n different wavelengths from λ1 to λn, n opticaldividers 291_1 to 291_n which divide into two the optical signals outputfrom the n optical senders 281_1 to 281_n, an optical multiplexer 211which multiplexes the n optical signals output from the optical dividers291_1 to 291_n and outputs them to the optical fiber (1) 62, and anoptical multiplexer 212 which multiplexes the n optical signals outputfrom the optical dividers 291_1 to 291_n and outputs them to the opticalfiber (2) 63.

[0069] The center node 21 a having one of the constitutions shown inFIGS. 4 to 6 for example divides an electrical signal, which is to betransmitted to the remote node 22 a, into two, and modulates two lightsources (the optical senders 251_1 and 252_1) which have an oscillatingfrequency of wavelength λ1 by using the two optical signals. The centernode 21 a also modulates one light source (the optical sender 281_1),and the optical dividers 291_1 divides the signal therefrom. One of thedivided signals is input to the optical fiber (1) 62, and the other isinput to the optical fiber (2) 63. Similarly, the center node 21 amodulates a light source having an oscillating frequency of wavelengthλ2 and generates one group of optical signals by using an electricalsignal which is to be transmitted to the remote node 23 a. One of thedivided signals is multiplexed with the optical signal having awavelength λ1 and is input to the optical fiber (1) 62, and the other ismultiplexed with the optical signal having a wavelength λ1 and is inputto the optical fiber (2) 63. The n optical wavelength divisionmultiplexing signals are input to the two optical fibers in the samemanner. In this network, two wavelengths are allocated to the opticalpath which joins the center node 21 a and the ONU 51 to 53. Onewavelength is allocated when transmitting from the center node 21 a toan ONU, and one wavelength is allocated when transmitting from the ONUto the center node 21 a. Therefore, when the total number of ONU in theregional network of this example is one hundred, two hundred wavelengthsare used.

[0070] One of the optical wavelength division multiplex signals whichare output from the center node 21 a is transmitted, for example,counterclockwise, and the other signal is transmitted clockwise. Thatis, the optical wavelength division multiplex signals are transmittedfrom the center node 21 a to the remote nodes 22 a, 23 a and 24 acounterclockwise at the optical fibers (1) 62 and 64, and clockwise atthe optical fibers (2) 61 and 63. Consequently, as shown in FIG. 2,optical wavelength division multiplex signals from two optical fibersare input to the remote nodes.

[0071] Optical amplifiers 101, 103, 102, and 104 which amplify theoptical signals which are input/output by using the optical fibers (1)and (2), and optical couplers (or optical circulators) 105 and 106 whichcouple light input from the optical fibers 65 or 68, comprising thenetwork (2) 12 a, to the optical signal which is transmitted along theoptical fiber (1) or the optical fiber (2), and divides the light whichis output to the optical fiber 66 or 67, are provided at the remote node24 a belonging to the network (1) 11 a. An optical switch 114 fordealing with severed fibers, an AWG (arrayed waveguide grating) 115comprising an optical multiplexer/de-multiplexer, optical amplifiers111, 111, 111, and 111, and an optical divider 113 are provided at theremote node 25 a which becomes the access node. FIG. 13 and FIG. 14(Table 1) show one example of the relationship between the wavelengthand input/output ports when using an AWG. For example, with regard tothe WDM (wavelength division multiplex) signals which are input from theinput port 1, a signal having a wavelength of λ1 is output from theoutput port 1. Conversely, when a signal having a wavelength of λ10 isinput from the output port 1, the signal having a wavelength of λ1 isoutput from the input port 7. Therefore, the wavelength divisionmultiplex signals can be de-multiplexed and multiplexed simultaneouslyby using the AWG. Here, λ1 to λ6, λ10 to λ15 represent different opticalwavelengths arranged in sequence according to wavelength.

[0072] The center node 21 a transmits signals constantly to the opticalfiber (1) 62 and the optical fiber (2) 63 toward the ONU in the network.As a consequence, the same signal is transmitted along two paths andinput to, for example, the optical switch 114 of the access node 25 a.The optical switch 114 shown in FIG. 2 is set so as to select theoptical signal which has been transmitted on the working fiber 67. Theoptical switch 114 selects only optical signals from the working fiber67, and outputs them to the optical multiplexer/de-multiplexer 115. Allof the for example one hundred optical signals which have beentransmitted from the center node 21 a toward all the ONU 51, 52, and 53,are input into the optical multiplexer/de-multiplexer 115. The opticalmultiplexer/de-multiplexer 115 selects only the corresponding wavelengthand transmits this signal to the corresponding ONU 51, 52, and 53.

[0073] A wavelength which has not been used in transmission from thecenter node 21 a is used for the optical signals to be transmitted fromthe ONU 51, 52, and 53 toward the center node 21 a. The signals from theONU 51, 52, and 53 are multiplexed by the opticalmultiplexer/de-multiplexer 115, and joined to the two opticaltransmission paths (represented by the solid and dotted lines) by usingthe optical divider 113 such as an optical coupler. After beingamplified by the respective optical amplifiers 111, the signals aretransmitted to the remote node 24 a. Since the remote node 24 a does notperform electrical processing, the signals from the ONU 51, 52, and 53are received at the center node 21 a from two paths comprising theoptical fiber (1) 64 and the optical fiber (2) 61. The center node 21 areceives the signals transmitted from the ONU 51, 52, and 53, and itsown transmitted signal, and extracts only the signals from the ONU byusing the optical de-multiplexers (201 and 202 in FIGS. 4 to 6). Thesignals from ONU which have been de-multiplexed by the opticalde-multiplexers (201 and 202) are converted to electrical signals by theoptical receivers (OR). From the converted electrical signals, aselector selects the electrical signals which corresponded to theworking fiber. The selected signals are electrically processed, anddistributed as signals to be transmitted within the network and signalsto be transmitted to a network at a higher level. That is, no electricalprocessing is carried out at the access node and the remote nodes.

[0074] Subsequently, the operation when a fiber has become severed atposition AA′ in the network (1) 11 a of FIG. 2 will be explained. Thecenter node 21 a is transmitting signals constantly toward the ONU inthe network on both the optical fibers (1) 62 and (2) 63. Therefore, thesame signal which has been transmitted along the two paths is input intothe optical switch 114 of the access node 25 a belonging to the network(2) 12 a. The optical switch 114 shown in FIG. 2 is set so as to selectthe optical signal transmitted on the working fiber 67. However, whenthe fiber has been severed at AA′, the optical switch 114 detects theseverance of an input signal and automatically switches so as to selectthe signal which has been transmitted on the protection fiber 66,represented by the dotted line. On the other hand, the signalstransmitted from the ONU 51, 52, and 53 to the center node 21 a aredivided by the optical divider 113 and always output to the working andprotection paths comprising the fibers 68 and 65. Since the remote node24 a does not perform electrical processing, the signals from the ONU51, 52, and 53 are normally received at the center node 21 a from twopaths comprising the optical fibers 61 and 64. When a fiber is severed,the selectors switch so that the signal which has been received from theworking fiber will be received from the protection fiber 64.

[0075] Similarly, in the case where there is an access node belonging tothe remote node (e.g. a remote node such as node 30 a, represented bythe chained line) provided downstream than to the signal beingtransmitted along the optical fiber (1), the optical switch switchesfrom working to protection. The signal is then transmitted by using theprotection path shown by the dotted line. In the case where there is anaccess node belonging to the remote node 22 a, provided upstream withregard to the signal being transmitted along the optical fiber (1), theoptical switch does not switch and the signal is transmitted along theworking path. At the center node 21 a, the selectors select eachdirection which a signal is input in at each wavelength, and the signalsare transmitted.

[0076] Subsequently, an example will be explained in the case where thefiber has been severed at point BB′ of the network (2) 12 a shown inFIG. 2.

[0077] At the access node 25 a which is connected to the remote node 24a, the optical switch 114 is switched to the direction shown by thedotted line. The switches at the other access nodes continue to inputthe signals in the working state, and consequently do not switch. At thecenter node 21 a, the selectors select each direction which a signal isinput in at each wavelength, and the signals are transmitted.

[0078] As described above, the optical signal is only electricallyprocessed at the center node 21 a and the ONU 51, 52, and 53, even whena fiber has been severed.

Embodiment 2

[0079]FIG. 3 shows an embodiment comprising a double ring. Thisembodiment differs from that shown in FIG. 2 in that (i) a plurality ofaccess nodes are connected to the remote node, (ii) the access nodes areconnected in a ring, and particularly (iii) this embodiment comprises anoptical band pass filter which prevents optical signals at thewavelengths allocated to the ONU 54, 55, and 56, which belong to thering network (4) 14 a comprising the remote node 23 a, from passingaround the ring network. An optical band pass filter 301 which passesonly wavelengths allocated for transmission from the remote node 23 a tothe center node 21 a, and optical band pass filter 302 which passes onlywavelengths allocated for transmission from the center node 21 a to theremote node 23 a, are connected to the input and output terminals of theoptical coupler 105 in the remote node 23 a. In addition, the opticalband pass filters 301 and 302 are connected to the input and outputterminals of an optical coupler 106. The constitution of these, and theoperation of the optical switches at the access nodes in the case wherethe fiber becomes severed at the point AA′, are the same as in the firstembodiment. The constitution of the center node 21 a is the same as thatshown in FIGS. 4 to 6. Incidentally, the remote node 23 a comprises thesame elements as the internal constitution of the remote node 24 a shownin FIG. 2. As shown by the access node (2) 27 a, the optical amplifiers111, 111, 111, and 111 in the access nodes (1) 26 a to (3) 28 a arearranged so that the optical fiber transmission paths form a ring, inthe same manner as the remote node 23 a.

[0080] Subsequently, the operation in the case where the fiber hasbecome severed at point BB′ will be explained. The optical switch at theaccess node (3) 28 a does not switch, since communication is possible byusing the working fiber shown by the solid line. Furthermore, theselector which corresponds to the wavelength allocated to the accessnode (3) 28 a does not switch at the center node 21 a. On the otherhand, at the access nodes (2) 27 a and (1) 26 a which are provideddownstream than the access node (3) 28 a, the optical signal from theworking fiber is severed. Consequently, the optical switch 114 switchesto the protection fiber shown by the dotted line. The signals from theONU 54, 55, and 56 are transmitted along the protection fiber to thecenter node 21 a via the remote node 23 a. At the center node 21 a, theselector selects a signal which corresponds to the signal from theprotection fiber. This has no effect on the access nodes correspondingto the remote node 22 a and the remote node 24 a.

[0081] In this network, the optical signal is electrically processedonly at the center node and the ONU, even when a fiber has been severed.

Embodiment 3

[0082]FIG. 7 shows an embodiment comprising a two-fiber unidirectionalring. This embodiment is characterized in that the transmissiondirection of the optical signal from the center node 21 a to the remotenode 24 b (corresponding to the remote node 24 a of FIG. 2) is the sameas the transmission direction of the optical signal from the remote node24 b to the center node 21 a. FIG. 7 shows the constitution of theremote node 24 b and the access node 25 a at this time. Thisconstitution differs from that shown in FIG. 2 in that the up and downsignals from the access node 25 a are input to identical opticalcouplers 105 b and 106 b, provided at the remote node 24 b. The opticalfibers 67 b and 68 b (corresponding to the optical fibers 67 and 68 ofFIG. 2) are connected to the optical coupler 105 b, and the opticalfibers 65 b and 66 b (corresponding to the optical fibers 65 and 66 ofFIG. 2) are connected to the optical coupler 106 b. Here, the network(1) 11 b comprising the center node 21 a is arranged as a two-fiberunidirectional ring which corresponds to the network (1) 11 a of FIG. 2.

[0083] The operation when the optical fiber has been severed at pointAA′ will be explained. The optical signal from the optical fiber (1) 62can be received at the access nodes which are connected at a lower levelthan the remote nodes 22 b and 23 b (corresponding to the remote nodes22 a and 23 a of FIG. 2). Therefore, the optical switches which areprovided at the access nodes do not switch to the protection fiber. Intransmitting from the access node to the center node 21 a, an opticaldivider, comprising an optical coupler or the like, divides the signalinto two. The optical fiber (2) 61 is the protection fiber, and connectsone of the divided signals to the center node 21 a. The selector at thecenter node 21 a selects the signal received from the optical fiber (2)61. On the other hand, at the access node 25 a which is connected at alower level than the remote node 24 b, the optical signal becomessevered. Consequently, the optical switch 114 switches to the protectionsystem. In transmitting from the access node 25 a to the center node 21a, only the signal in the divided output of the opticalmultiplexer/de-multiplexer 115 which is connected to the optical fiber(1) 64 is transmitted counterclockwise along the optical fiber (1) 64 tothe center node 21 a. Since the center node 21 a has already selectedthe signal which was received from the optical fiber (1) 64, theselectors do not change its signal selection. Therefore, when the cableis severed at the point AA′, the transmission path of the two-fiberunidirectional ring network becomes the same as that in thebi-directional ring.

[0084] Subsequently, the operation when the cable is severed at thepoint BB′ will be explained. At the access node 25 a connected to theremote node 24 b, the optical switch 114 switches to the protectionsystem when the cable is severed. The signal from the access node 25 ato the remote node 24 b is transmitted along the fibers 65 b and 66 b,represented by dotted lines, and connects to the protection opticalfibers (2) 61 and 63 in the remote node 24 b. The signal is transmittedclockwise along the optical fibers (2) 61 and 63. The selector at thecenter node 21 a selects the signal which is received from the opticalfiber (2) 61. The signals corresponding to the remote nodes 22 b and 23b are not switched by the access nodes connected thereto, nor are theysubject to the change in signal selection by the selectors at the centernode 21 a.

[0085] In the network described above, the optical signal is onlyelectrically processed at the center node and the ONU even in the casewhere a fiber has been severed.

Embodiment 4

[0086] In the constitution shown in FIG. 8, the network (1) 11 b(corresponding to the network (1) 11 a of FIG. 3) comprising the centernode 21 a is a two-fiber unidirectional ring, and the lower levelnetwork (4) 14 b (corresponding to the network (4) 14 a of FIG. 3) alsocomprises a ring. This constitution differs from that shown in FIG. 3 inthat the up and down signals from the access node are input to identicaloptical couplers 105 b and 106 b, provided at the remote node 23 b(corresponding to the remote node 23 a of FIG. 3), as in the remote node24 b of FIG. 7. Another important difference to FIG. 7 is that theprovision of band-pass filters which prevent optical signals at thewavelengths allocated to the ONU 54, 55, and 56 in the ring network (4)14 b comprising the remote node 23 b, from passing around the ringnetwork. An optical band-pass filter 301 which passes only wavelengthsallocated for transmission from the remote node 23 b to the center node21 a, and optical band-pass filter 302 which passes only wavelengthsallocated for transmission from the center node 21 a to the remote node23 b, are connected to the input and output terminals of the opticalcoupler 105 b and the optical coupler 106 b. As in the previousembodiments, the optical switches of the access nodes and the selectorsof the center nodes are normally set so as to select the signals fromthe working fiber, shown by the solid line.

[0087] The operation when the optical fiber has been severed at thepoint AA′ will be explained. The optical signal from the optical fiber(1) 62 can be received at the access node which is connected at a lowerlevel than the remote node 22 b. Therefore, the optical switches whichare provided at the access node lower level than the remote node 22 b donot switch to the protection fiber. In transmitting from the access nodeto the center node 21 a, an optical divider, comprising an opticalcoupler or the like, divides the signal into two. One of the dividedsignals is transmitted clockwise to the center node 21 a along theoptical fiber (2) 61, which comprises the protection fiber. The selectorat the center node 21 a selects the signal received from the opticalfiber (2) 61. On the other hand, at the access node which is connectedat a lower level than the remote nodes 23 b and 24 b, the optical signalbecomes severed. Consequently, the optical switch switches to theprotection system. In transmitting from the access node to the centernode, only the signal in the divided output of the opticalmultiplexer/de-multiplexer which is connected to the optical fiber (1)64 is transmitted counterclockwise along the optical fiber (1) 64 to thecenter node 21 a. Since the center node 21 a has already selected thesignal which is received from the optical fiber (1) 64, the selectors donot change its signal selection. Therefore, when the cable is severed atthe point AA′, the transmission path of the two-fiber unidirectionalring network becomes the same as that in the bi-directional ring.

[0088] Subsequently, the operation when the cable is severed at thepoint BB′ will be explained. Since the optical signal is not cut-off atthe access node (3) 28 a connected to the remote node 23 b, the opticalswitch 114 does not switch. Since the signal is transmitted to thecenter node 21 a along the working fiber, the selector in the receivingsection of the center node 21 a does not change its signal selection. Onthe other hand, the optical signal is cut-off at the access nodes (2) 27a and (1) 26 a. Therefore, when the cable is severed, the optical switch114 switches to the protection system, and the optical signal isreceived from the protection fiber. The signals from the access nodes 26a and 27 a to the remote node 23 b are transmitted along the fibersrepresented by dotted lines, and connect to the protection opticalfibers (2) 61 and 63 through the optical coupler 106 b in the remotenode 23 b. The signals are transmitted clockwise along the opticalfibers (2) 61 and 63. The selector at the center node 21 a selects thesignal which was received from the optical fiber (2) 61. The signalscorresponding to the remote nodes 22 b and 24 b are not switched at theaccess nodes connected thereto, nor are they subject to the change insignal selection by the selectors at the center node 21 a.

[0089] In the network described above, the optical signal is onlyelectrically processed at the center node and the ONU even in the casewhere a fiber has been severed.

Embodiment 5

[0090]FIG. 9 shows an embodiment in which an optical signal is convertedto an electrical signal by using transponders (in FIG. 9, opticalamplifiers/senders) 121, 121, 121, and 121, in a remote node 24 ccorresponding to the remote node 24 a of FIG. 2. At the remote node 24c, optical de-multiplexers in the transponders 121, 121, 121, and 121de-multiplex only the wavelengths which correspond to the ONU 51, 52,and 53 belonging to lower levels. Signals at each wavelength arereceived, equalized, identified, reproduced, and retransmitted usingappropriate wavelengths. Signals from the access node 25 a are similarlyprocessed, converted to predetermined wavelengths, and transmitted tothe center node 21 a. Signals can be multiplexed and de-multiplexed byusing an optical multiplexer/de-multiplexer such as an AWG.

[0091] In the example shown in FIG. 9, remote nodes 26 c and 27 c havethe same constitution as the remote node 24 c, and are provided in thering network comprising the center node 21 a and the remote node 24 c. Acenter node 71 and a plurality of remote nodes 72, 72, . . . areprovided in the higher level ring network comprising the center node 21a.

Embodiment 6

[0092]FIG. 10 shows another embodiment in which an optical signal isconverted to an electrical signal by using transponders 121, 121, 121,and 121, in a remote node 23 c corresponding to the remote node 23 a ofFIG. 3. The constitution is the same as that shown in FIG. 9, with theexception that the access nodes (1) to (3) and the remote node 23 c areconnected in a ring.

[0093] In the example shown in FIG. 10, remote nodes 22 c and 24 c havethe same constitution as the remote node 23 c, and are provided in thering network comprising the center node 21 a and the remote node 23 c. Acenter node 71 and a plurality of remote nodes 72, 72, . . . areprovided in the higher level ring network comprising the center node 21a.

Embodiment 7

[0094]FIG. 11 shows an embodiment in which communication between theaccess node 25 c (corresponding to the access node 25 a of FIG. 2) andthe ONU 51, 52, and 53, is doubled by using radio communication (radioreceiver/sender 130, 131, 132, and 133). When communication is doubledby using radio, all the paths which join the ONU 51, 52, and 53 to thecenter node 21 a can be doubled inexpensively. FIG. 11 shows only atwo-fiber unidirectional ring, but this constitution can be applied toall networks in the embodiments of this invention. The operations whenthe optical cable becomes severed at points AA′ and BB′ (not shown inFIG. 11) are the same as those described in the first embodiment.

Embodiment 8

[0095]FIG. 12 shows an embodiment in which the opticalmultiplexer/de-multiplexer 115 a is provided at a remote terminal 29near the user, instead of at the access node 25 d (corresponding to theaccess node 25 a of FIG. 2). In this embodiment, the constitution of thenetwork above the access node 25 d can be applied in all of theembodiments of this invention. By providing the opticalmultiplexer/divider nearer to the ONU, the cost of establishing the pathcan be reduced. The operations when the optical cable becomes severed atpoints AA′ and BB′ (not shown in FIG. 12) are the same as thosedescribed in the first embodiment.

Embodiment 9

[0096] FIGS. 15 to 17 show an embodiment wherein, at the remote nodes(offices) which belong to the lower level ring network comprising theaccess node and the higher level ring network, both ends of twoloop-like optical fibers (one working fiber and one protection fiber)which join the access nodes belonging to the lower level ring networkare open (specifically, between the optical terminators 1509 and 1510 ofthe working fiber, and between the optical terminators 1609 and 1610 ofthe protection fiber). Instead of providing an opticalmultiplexer/de-multiplexer having wavelength selectability at the accessnodes or the above remote nodes (offices), the ONU themselves have anoptical de-multiplexing function. Further, the wavelength divisionmultiplexing signals which are transmitted along the two optical fibersused in the ring networks are all bi-directional, and bi-directionaloptical amplifiers are used in the remote nodes and the access nodes.(This embodiment corresponds to Claims 11 and 12.) FIGS. 15 to 17 show acase where, in the higher level ring network comprising the center node21 b, the optical signal from the center node 21 b to the remote node1504 is transmitted in the opposite direction to the optical signal fromthe remote node 1504 to the center node 21 b, i.e. a bi-directionalring. In FIG. 15, fibers 1501, 1503, 1511, 1513, 1514, 1516, 1522, and1524 are working fibers, and in FIG. 16, fibers 1601, 1603, 1611, 1613,1614, 1616, 1622, and 1624 are protection fibers. Firstly, signaltransmission on the working fibers 1501, 1503, 1511, 1513, 1514, 1516,1522, and 1524 will be explained. A signal is transmittedcounterclockwise from the center node 21 b to the remote node 1504. Afiber coupler 1505 is provided at the remote node 1504, and divides thesignal, which is then transmitted to the lower level ring networkcomprising the access node 1517. In the access node 1517 shown in FIG.17, the received optical wavelength division multiplexing signal isdivided by the fiber coupler 1518 and received. One end of the fibercoupler 1518 functions as an optical terminator 1520. The opticalwavelength division multiplexing signal which was divided at the accessnode 1517 is led to an optical switch 1702 by a circulator 1701, andthen led to a star coupler 1709 by another optical circulator 1705. Thestar coupler 1709 distributes the signal to the ONU 1706, 1707, and1708. The ONU 1706, 1707, and 1708 de-multiplex and receive only signalsat wavelengths allocated to the ONU. It is a major feature of thisembodiment that the ONU 1706, 1707, and 1708 have the ability tode-multiplex signals. No optical de-multiplexers having wavelengthselectability, such as an AWG, are provided in the access node 1517 andthe remote node 1504. Instead, the ONU 1706, 1707, and 1708 themselvesare able to de-multiplex wavelengths.

[0097] In this example, the optical circulators 1701, 1703, and 1705comprise optical circuits in which an optical signal which has beeninput from a port (1) is output from a port (2), an optical signal whichhas been supplied from the port (2) is output from a port (3), and anoptical signal which has been supplied from the port (3) is output fromthe port (1).

[0098] As in the embodiments described above, when transmitting from theONU 1706, 1707, and 1708 to the center node 21 b, the ONU 1706, 1707,and 1708 use predetermined wavelengths. In contrast to the aboveembodiments, the signals transmitted from the ONU 1706, 1707, and 1708are multiplexed by the star coupler 1709. Thereafter, an opticalcirculator 1705, an optical divider 1704, and another optical circulator1701 transmit the signal in the opposite direction to that received onthe optical fiber.

[0099] Another important feature of this embodiment is that both ends ofthe looped optical fibers 1514, 1516, 1522, and 1524 which join theaccess nodes 1515, 1517, and 1523, to the remote node 1504 are opened byoptical terminators 1509 and 1510 in the remote node 1504. This is toprevent the optical signals from passing around the lower level ringnetwork which the access node 1517 belongs to.

[0100] Subsequently, in the protection system shown in FIG. 16, thesignals are transmitted in the opposite direction to that in FIG. 15.The open ends of the loop in the remote node 1504 (optical terminators1609 and 1610) are also provided at opposite positions. The operation ofthe optical switch 1702 provided at the access node 1517 is the same asthe embodiments described above. The access node in this embodimentcomprises an optical circulator, but an optical coupler mayalternatively be used.

[0101] In FIGS. 15 to 17, reference numerals 1505, 1518, 1605, and 1618represent two-by-two optical fiber couplers, reference numerals 1506,1507, 1508, 1519, 1521, and 1606, 1607, 1608, 1619, and 1621 representbi-directional optical amplifiers, reference numerals 1502 and 1512represent remote nodes, reference numerals 1515 and 1523 representaccess nodes, and reference numeral 1620 represents an opticalterminator. The solid-line arrows show the direction of the opticalsignals which are transmitted from the center node toward the ONU, andthe broken-line arrows show the direction of the optical signals whichare transmitted from the ONU toward the center node. The operations inthe cases where the optical cable becomes severed at the points AA′ andBB′ (not shown in FIGS. 15 to 17) are the same as that already describedin the second embodiment.

[0102]FIG. 18 shows one example of the constitution of the center node21 b according to this embodiment. In the constitution of the centernode 21 b shown in FIG. 18, parts which are identical to those in theconstitution of the center node 21 a shown in FIG. 4 are represented bythe same reference numerals, and will not be explained further. Thecenter node 21 b shown in FIG. 18 comprises an optical terminator 1801which terminates the working fiber 1501 shown in FIG. 15, an opticalcirculator 1802 connected to the protection fiber 1601 of FIG. 16, anoptical amplifier 1803 having an input terminal connected to the port(3) of an optical circulator 1802, an optical amplifier 1804 having anoutput terminal connected to the port (1) of the optical circulator1802, an optical terminator 1805 which terminates the protection fiber1613 shown in FIG. 16, an optical circulator 1806 which is connected tothe working fiber 1513 of FIG. 15, an optical amplifier 1807 having aninput terminal connected to the port (3) of the optical circulator 1806,and an optical amplifier 1808 having an output terminal connected to theport (1) of the optical circulator 1806. In this case, the output of theoptical amplifier 1803 connects to the input of the opticalde-multiplexer 202, the input of the optical amplifier 1804 connects tothe output of the optical multiplexer 212, the output of the opticalamplifier 1807 connects to the input of the optical de-multiplexer 201,and the input of the optical amplifier 1808 connects to output of theoptical multiplexer 211. Incidentally, the optical amplifiers 1803,1804, 1807, and 1808 need only be provided as necessary.

Embodiment 10

[0103]FIGS. 19 and 20 show embodiments corresponding to Claims 11 and12. FIGS. 19 and 20 show the case where, in the higher level ringnetwork comprising the center node 21 c, the direction of the opticalsignal which transmits data from the center node 21 c to the remote node1904 is the same as the direction of the optical signal which transmitsdata from the remote node 1904 to the center node 21 c, i.e. the networkis a unidirectional ring. FIG. 19 shows a working fiber, and FIG. 20shows a protection fiber.

[0104] As in FIGS. 15 to 17, solid-line arrows and broken-line arrowsare used to represent examples of the directions of signals transmittedfrom the center node 21 c via the remote node #2 (1904) to the ONUbelonging to the access node 2 (1517). FIGS. 19 and 20 differ from FIGS.15 to 17 in that (i) bi-directional optical amplifiers are not needed inthe higher level ring network, and (ii) the remote node 1904 comprisesoptical circulators 1909 and 2009. FIG. 21 shows an example of theconstitution of the center node used here.

[0105] In FIGS. 19 and 20, reference numerals 1905 and 2005 representtwo-by-two optical fiber couplers, reference numerals 1907 and 2007represent bi-directional optical amplifiers, reference numerals 1906,1908, 2006, and 2008 represent (unidirectional) optical amplifiers,reference numerals 1910 and 2010 represent optical terminators, andreference numerals 1909 and 2009 represent optical circulators whichconnect the port (1) to the optical fiber couplers 1905 and 2005, andconnect the port (2) to the port (3). The operations in the cases wherethe optical cable becomes severed at the points AA′ and BB′ (not shownin FIGS. 19 to 20) are the same as that already described in the secondembodiment. The center node 21 c shown in FIG. 21 comprises an opticalamplifier 2101 which inputs signals from the working fiber 1501 andoutputs signals to the optical de-multiplexer 202, an optical amplifier2102 which outputs signals to the protection fiber 1601 and inputssignals from the optical multiplexer 212, an optical amplifier 2103which inputs signals from the optical fiber 1613 and outputs signals tothe optical de-multiplexer 201, and an optical amplifier 2104 whichoutputs signals to the optical fiber 1513 and inputs signals from theoptical multiplexer 211. The optical amplifiers 2101 to 2104 may beprovided where necessary.

Embodiment 11

[0106]FIG. 22 shows an embodiment comprising a three-layered opticalnetwork in which the highest level network is a ring network comprisingone center node and two or more remote nodes, which are joined by fouroptical fibers. The intermediate level network comprises a ring networkhaving a node belonging to the higher level network as its center node.Access nodes belonging to the ring network are joined by four opticalfibers. The lowest level network comprises a star network centeredaround an access node, which multiplexes traffic from ONU. The ONU andaccess node are each directly joined by one optical fiber. The centernode belonging to the highest level network and the ONU establish adirect communication path by using lights of different wavelengths. Theoptical signals are not electrically processed, but are amplified,branched, or routed at the remote nodes and the access node providedtherebetween. In addition, at the node belonging to the intermediatelevel ring network, both ends of the four looped optical fibers (two ofthe optical fibers corresponding to working fibers, and twocorresponding to protection fibers) which join together the access nodesbelonging to the lower level ring network, are open (between opticalterminators 2203 g and 2203 h, and between 2203 i and 2203 j on theworking fiber; between optical terminators 2203 q and 2203 r, andbetween 2203 s and 2203 t on the protection fiber). Furthermore, theaccess nodes and the remote nodes do not comprise opticalmultiplexer/de-multiplexers having the ability to select wavelengths.Instead, the ONU themselves having a wavelength de-multiplexingfunction. (This embodiment corresponds to Claims 13 and 14.) The block2201 a enclosed by the chain line comprises two working fibers, and theblock 2201 b comprises two protection fibers. The access node 2206connects to four optical fibers. In the case shown in FIG. 22, theaccess node 2206 comprises an optical coupler 2206 i, but an opticalcirculator may be used instead, as shown in FIG. 17. FIG. 22 shows abi-directional ring network comprising two working fibers and twoprotection fibers, the signals to the ONU being transmitted in theopposite direction from signals transmitted from the ONU. Thisembodiment is characterized in that (i) both ends of the optical fiberswhich connect the access node in a ring are open at the remote node,preventing the signals from passing around the loop, and (ii) no opticalmultiplexer/de-multiplexer having wavelength selectability is used atthe remote node and the access node. Instead, the ONU are able to selectwavelengths for transmitting and receiving.

[0107] In FIG. 22, the solid lines represent optical fibers which areused in communication between the center node 21 d and the access node2206. The same applies in an embodiment subsequently described in FIG.24. Reference numeral 21 d represents the center node, referencenumerals 2202, 2203, 2204 represent remote nodes, reference numerals2212 a, 2213 a, 2219 a, and 2220 a represent ring-shaped optical fibersfor working, reference numerals 2214 a, 2215 a, 2217 a, and 2218 arepresent ring-shaped optical fibers for working, reference numerals2212 b, 2213 b, 2219 b, and 2220 b represent ring-shaped optical fibersfor protection, reference numerals 2214 b, 2215 b, 2217 b, and 2218 brepresent ring-shaped optical fibers for protection, reference numerals2203 a and 2203 b represent two-by-two fiber couplers, referencenumerals 2203 c, 2203 d, 2203 e, and 2203 f represent opticalamplifiers, reference numerals 2203 g, 2203 h, 2203 i, and 2203 jrepresent optical terminators where the fiber loop is open, referencenumerals 2203 k and 2203 l represent two-by-two fiber couplers,reference numerals 2203 m, 2203 n, 2203 o, and 2203 p represent opticalamplifiers, and reference numerals 2203 q, 2203 r, 2203 s, and 2203 trepresent optical terminators where the fiber loop is open. Referencenumerals 2205, 2206, and 2207 represent access nodes, reference numeral2208 represents a star coupler, reference numerals 2209, 2210, and 2211represent ONU, reference numerals 2212 and 2213 represent protectionoptical fibers, reference numerals 2206 a, 2206 b, 2206 j, and 2206 krepresent two-by-two fiber couplers, reference numerals 2206 c, 2206 d,2206 e, 2203 f, 2206 l, 2206 m, 2206 n, and 2206 o represent opticalamplifiers, reference numeral 2206 g represents an optical switch, andreference numerals 2206 h represents an opticalmultiplexer/de-multiplexer. The solid-line arrows show the direction ofthe optical signals which are transmitted from the center node towardthe ONU, and the broken-line arrows show the direction of the opticalsignals which are transmitted from the ONU toward the center node. Theoperations in the cases where the optical cable becomes severed at thepoints AA′ and BB′ (not shown in FIG. 22) are the same as that alreadydescribed in the second embodiment.

[0108]FIG. 23 shows one example of the constitution of the center nodein this embodiment. The center node 21 d comprises an optical amplifier2301 which inputs signals from the ring-shaped optical fiber 2215 b andoutputs signals to the optical de-multiplexer 202, an optical amplifier2302 which outputs signals to the ring-shaped optical fiber 2214 b andinputs signals from the optical multiplexer 212, an optical amplifier2303 which inputs signals from the optical fiber 2212 a and outputssignals to the optical de-multiplexer 201, and an optical amplifier 2304which outputs signals to the optical fiber 2213 a and inputs signalsfrom the optical multiplexer 211. The optical amplifiers 2301 to 2304may be provided where necessary.

Embodiment 12

[0109]FIG. 24 shows an embodiment (corresponding to Claims 15 and 16)which provides an optical wavelength division multiplexing networkcomprising at least three layers. The highest level network comprises aring network having at least one center node and two or more remotenodes which are joined by two optical fibers. The intermediate levelnetwork comprises a ring network having a node belonging to the higherlevel network as its center node. Access nodes belonging to the ringnetwork are joined by four optical fibers. The lowest level networkcomprises a star network centered around an access node, whichmultiplexes traffic from ONU. The ONU and the access node are directlyjoined by one optical fiber. The center node belonging to the highestlevel network and the ONU establish a direct communication path by usinglights of different wavelengths. The optical signals are notelectrically processed at the remote nodes and the access nodes providedbetween the center node and the ONU. Instead, only the optical signalsare amplified, divided, or routed. At a node (an office) belonging tothe intermediate level ring network, one end of the four looped opticalfibers (two for working, and two for protection) which join the accessnodes belonging to the lower level ring network, are open (opticalterminators 2403 d and 2403 e on the working fiber; optical terminators2403 i and 2403 j on the protection fiber). Furthermore, the accessnodes and the remote nodes (offices) do not comprise opticalmultiplexer/de-multiplexers having the ability to select wavelengths.Instead, the ONU themselves having a wavelength de-multiplexingfunction. The block 2401 a enclosed by the chain line comprises theworking fiber, and the block 2401 b comprises the protection fiber. Fouroptical fibers connect the access nodes shown in the blocks 2401 a and2401 b. In FIG. 24, the access node 2406 comprises an optical coupler2406 i, but an optical circulator may be used instead, as shown in FIG.17. FIG. 24 shows a unidirectional ring network comprising twoworking/protection fibers, the signals to the ONU being transmitted inthe same direction as signals transmitted from the ONU.

[0110] In FIG. 24, reference numeral 21 e represents the center node,reference numerals 2402, 2403, 2404 represent remote nodes, referencenumerals 2412 a, 2418 a, and 2419 a represent ring-shaped optical fibersfor working, reference numerals 2414 a, 2417 a, and 2420 a representring-shaped optical fibers for working, reference numerals 2412 b, 2418b, and 2419 b represent ring-shaped optical fibers for protection,reference numerals 2414 b, 2417 b, and 2420 b represent ring-shapedoptical fibers for protection, reference numerals 2403 a and 2403 frepresent two-by-two fiber couplers, reference numerals 2403 b, 2403 c,2403 g, and 2403 h represent optical amplifiers, reference numerals 2403d, 2403 e, 2403 i, and 2403 j represent optical terminators where thefiber loop is open, reference numerals 2405, 2406, and 2407 representaccess nodes, reference numeral 2408 represents a star coupler,reference numerals 2409, 2410, and 2411 represent ONU, referencenumerals 2412 and 2413 represent optical fibers for working, referencenumerals 2406 a, 2406 b, 2406 j, and 2406 k represent two-by-two fibercouplers, reference numerals 2406 c, 2406 d, 2406 e, 2406 f, 2406 l,2406 m, 2406 n, and 2406 o represent optical amplifiers, referencenumeral 2406 g represents an optical switch, and reference numerals 2406h represents an optical multiplexer/de-multiplexer. The solid-linearrows show the direction of the optical signals which are transmittedfrom the center node toward the ONU, and the broken-line arrows show thedirection of the optical signals which are transmitted from the ONUtoward the center node. Incidentally, the constitution of the centernode 21 e may be the same as, for example, that in FIG. 23. Theoperations in the cases where the optical cable becomes severed at thepoints AA′ and BB′ (not shown in FIG. 24) are the same as that alreadydescribed in the second embodiment.

Embodiment 13

[0111]FIG. 26 shows an embodiment of a network which corresponds toClaim 20. Two fibers which are usually used as working fibers are shownon the left side of FIG. 26, and the remaining two fibers which are usedas protection fibers are shown on the right side. That is, the block2601 a enclosed by the chain line comprises the two working fibers, andthe block 2601 b comprises the two protection fibers. Four opticalfibers connect the access node 2606.

[0112] In FIG. 26, the solid lines on the left side (working) of thediagram, and the thick dotted lines on the right (protection) side ofthe diagram represent optical fibers which are used in communicationsbetween the center node 21 f and the access node 2606. The same appliesin embodiments shown in FIGS. 27 and 28, which will be explained later.Reference numeral 21 f represent the center node, reference numerals2602, 2603, an 2604 represent the remote nodes which, with the centernode 21 f, comprise the higher level ring network, reference numerals2612 a, 2613 a, 2614 a, 2615 a, 2617 a, 2618 a, 2619 a, and 2620 arepresent ring optical fibers for working, reference numerals 2612 b,2613 b, 2614 b, 2615 b, 2617 b, 2618 b, 2619 b, and 2620 b representring optical fibers for protection, reference numerals 2603 a and 2603 brepresent two-by-two couplers, reference numerals 2603 c, 2603 d, 2603e, 2603 f, 2603 u, and 2603 v represent optical amplifiers, referencenumerals 2603 g, 2603 h, 2603 i, and 2603 j represent opticalterminators where the fiber loop is open, reference numerals 2603 k and2603 l represent two-by-two fiber couplers, reference numerals 2606 m,2606 n, 2606 o, 2606 p, 2606 w, and 2606 x represent optical amplifiers,reference numerals 2603 q, 2603 r, 2603 s, and 2603 t represent opticalterminators where the fiber loop is open. Reference numerals 2605, 2606,and 2607 represent access nodes which, with the remote node 2603,comprise the lower level ring network, reference numerals 2609, 2610,and 2611 represent ONU, reference numerals 2612 and 2613 representoptical fibers for protection, reference numerals 2606 a, 2606 b, 2606j, and 2606 k represent two-by-two fiber couplers, reference numerals2606 c, 2606 d, 2606 e, 2606 f, 2606 l, 2606 m, 2606 n, 2606 o, 2606 p,and 2606 q represent optical amplifiers, reference numeral 2606 grepresents an optical divider, reference numerals 2606 h represents anAWG, and reference numerals 2606 i represents an optical switch.Incidentally, the constitution of the center node 21 f may be the sameas, for example, that in FIG. 23.

[0113]FIG. 26 shows communication from the center node 21 f, via theremote node 2603, to the ONU 2609, 2610, and 2611 belonging to theaccess node 2606. The working network shown on the left side of FIG. 26will be explained. The center node 21 f allocates wavelengths to the ONU2609, 2610, and 2611 belonging to the access node 2606, and transmitssignals to the remote nodes 2602, 2603, and 2604 in the higher levelnetwork by using the fiber 2613 a. At the remote nodes 2602, 2603, and2604, the optical couplers branch the optical wavelength divisionmultiplexing signals which have been transmitted. Taking the remote node2603 by way of example, the remote node 2603 transmits the branchedsignals to the access nodes 2605, 2606, and 2607 belonging to the lowerlevel ring network. At the access nodes 2605, 2606, and 2607 belongingto the lower level ring network, the optical wavelength divisionmultiplexing signals transmitted from the center node 21 f are dividedby using an optical coupler. Taking the access node 2606 by way ofexample, the optical wavelength division multiplexing signals from thecenter node 21 f which have been divided by the optical coupler 2606 aare divided by the AWG 2606 h, separated into the allocated wavelengths,and received at the ONU 2609, 2610, and 2611. The ONU 2609, 2610, and2611 transmit to the AWG 2606 h by using the same wavelength as thatreceived. The AWG 2606 h is connected to each ONU 2609, 2610, and 2611by two optical fibers, one for receiving down signals and one for upsignals to the center node. The wavelengths allocated to the ONU 2609,2610, and 2611 are such that they are not output from adjacent outputports of the AWG 2606 h. The optical fibers which transmit the up signalare connected to ports adjacent to the down signal output port of theAWG 2606 h. As a consequence, the signals from the ONU 2609, 2610, and2611 are multiplexed and transmitted from the access node 2606 to thecenter node 21 f. The signals transmitted from the access node 2606 tothe center node 21 f are coupled by the optical coupler 2606 b, andtransmitted to the remote node 2603. In the same way, up signals fromthe ONU belonging to the lower level network are multiplexed at theremote node 2603, and transmitted by using the optical fiber 2612 a tothe center node 21 f. In this embodiment, the optical wavelengthdivision multiplexing signals are transmitted on the fibers 2613 a and2612 a in the higher level network in opposite directions.

[0114] At the access node 2606, the optical switch 2606 i is provided inthe input section for the down signal to the AWG 2606 h, in order toswitch to the protection system in the case where the fiber becomessevered. Furthermore, the optical divider 2606 g is provided in theoutput section for the up signal from the AWG 2606 h, in order totransmit the up signal on both the working and protection fibers. Asalready explained, the working and protection signals are selected inthe center node 21 f.

[0115] This embodiment is characterized in that (i) there is noswitching at the remote nodes when the fiber becomes severed, and (ii)the optical signal is not electrically processed at the nodes (offices)provided between the ONU and the center node. Incidentally, theoperations in the cases where the optical cable becomes severed at thepoints AA′ and BB′ (not shown in FIG. 26) are the same as that alreadydescribed in the second embodiment.

Embodiment 14

[0116]FIG. 27 shows an embodiment which corresponds to Claim 20. Thisembodiment differs from that shown in FIG. 26 in that, in the higherlevel ring network, the up and down signals are transmitted in the samedirection.

[0117] In FIG. 27, the block 2701 a enclosed by the chain line comprisesthe two working fibers, and the block 2701 b comprises the twoprotection fibers. Four optical fibers connect the access node 2706. Thereference numeral 21 g represents the center node, reference numerals2702, 2703, and 2704 represent remote nodes comprising, with the centernode 21 g, the higher level ring network, reference numerals 2712 a,2713 a, 2714 a, 2715 a, 2717 a, 2718 a, 2719 a, and 2720 a representring-shaped optical fibers for working, reference numerals 2712 b, 2713b, 2714 b, 2715 b, 2717 b, 2718 b, 2719 b and 2720 b representring-shaped optical fibers for protection, reference numerals 2703 a and2703 b represent two-by-two fiber couplers, reference numerals 2703 c,2703 d, 2703 e, 2703 f, 2703 u, and 2703 v represent optical amplifiers,reference numerals 2703 g, 2703 h, 2703 i, and 2703 j represent opticalterminators where the fiber loop is open, reference numerals 2703 k and2703 l represent two-by-two fiber couplers, reference numerals 2703 m,2703 n, 2703 o, 2703 p, 2703 w, and 2703 x represent optical amplifiers,and reference numerals 2703 q, 2703 r, 2703 s, and 2703 t representoptical terminators where the fiber loop is open. Reference numerals2705, 2706, and 2707 represent access nodes which, with the remote node2703, comprise the lower level ring network, reference numerals 2709,2710, and 2711 represent ONU, reference numerals 2712 and 2713 representoptical fibers for protection, reference numerals 2706 a, 2706 b, 2706j, and 2706 k represent two-by-two fiber couplers, reference numerals2706 c, 2706 d, 2706 e, 2706 f, 2706 l, 2706 m, 2706 n, 2706 o, 2706 pand 2706 q represent optical amplifiers, reference numeral 2706 grepresents an optical divider, reference numeral 2706 h represents anAWG, and reference numeral 2706 i represents an optical switch.Incidentally, the constitution of the center node 21 g may be the sameas, for example, the center node shown in FIG. 21. The operations in thecases where the optical cable becomes severed at the points AA′ and BB′(not shown in FIG. 27) are the same as that already described in thesecond embodiment.

Embodiment 15

[0118]FIG. 28 shows an embodiment corresponding to Claim 21. With theexception of the constitution of the access node, this embodiment isidentical to that of FIG. 26. In FIG. 28, the block 2801 a enclosed bythe chain line comprises the two working fibers, and the block 2801 bcomprises the two protection fibers. Four optical fibers connect theaccess node 2806. The reference numeral 21 h represents the center node,reference numerals 2802, 2803, and 2804 represent remote nodescomprising, with the center node 21 h, the higher level ring network,reference numerals 2812 a, 2813 a, 2814 a, 2815 a, 2817 a, 2818 a, 2819a, and 2820 a represent ring-shaped optical fibers for working,reference numerals 2812 b, 2813 b, 2814 b, 2815 b, 2817 b, 2818 b, 2819b and 2820 b represent ring-shaped optical fibers for protection,reference numerals 2803 a and 2803 b represent two-by-two fibercouplers, reference numerals 2803 c, 2803 d, 2803 e, 2803 f, 2803 u, and2803 v represent optical amplifiers, reference numerals 2803 g, 2803 h,2803 i, and 2803 j represent optical terminators where the fiber loop isopen, reference numerals 2803 k and 2803 l represent two-by-two fibercouplers, reference numerals 2803 m, 2803 n, 2803 o, 2803 p, 2803 w, and2803 x represent optical amplifiers, and reference numerals 2803 q, 2803r, 2803 s, and 2803 t represent optical terminators where the fiber loopis open. Reference numerals 2805, 2806, and 2807 represent access nodeswhich, with the remote node 2803, comprise the lower level ring network,reference numerals 2809, 2810, and 2811 represent ONU, referencenumerals 2812 and 2813 represent optical fibers for protection,reference numerals 2806 a, 2806 b, 2806 j, and 2806 k representtwo-by-two fiber couplers, reference numerals 2806 c, 2806 d, 2806 e,2806 f, 2806 l, 2806 m, 2806 n, 2806 o, 2806 p, and 2806 q representoptical amplifiers, reference numeral 2806 g represents an opticalswitch, reference numeral 2806 h represents an optical coupler,reference numeral 2806 i represents an optical coupler, and referencenumerals 2806 y and 2806 z represent star couplers. Incidentally, theconstitution of the center node 21 h may be the same as, for example,the center node shown in FIG. 23.

[0119] One feature of this embodiment is that, star couplers which arenot dependent on wavelength are used in distributing signals to the ONUbelonging to the access nodes, and in multiplexing signals from the ONU.This is effective when the number of ONU varies from access node toaccess node. Furthermore, the ONU in this embodiment must be capable ofselecting wavelengths. This embodiment also has the advantages that (i)there is no switching at the remote nodes when the fiber becomessevered, and (ii) the optical signal is not electrically processed atthe nodes (offices) provided between the ONU and the center node.Incidentally, the operations in the cases where the optical cablebecomes severed at the points AA′ and BB′ (not shown in FIG. 28) are thesame as that already described in the second embodiment.

[0120] According to the embodiments described above, it is possible toreduce initial expenditure when realizing a large-capacity accessservice by using ONU. Further, when increasing the number of ONU, onlythe transmission apparatuses at the center node need be increased,achieving an easily expandable network.

What is claimed is:
 1. An optical wavelength division multiplexingnetwork having a structure comprising at least two layers, a highestlevel network being a ring network which comprises at least one centernode and two or more remote nodes which are joined by at least twooptical fibers; in the case where the layered structure comprises threeor more layers, excepting the lowest level network the intermediatelevel network comprising a ring having said node belonging to thehighest level network as its center node, nodes belonging to said ringnetwork being joined by at-least two optical fibers; said lowest levelnetwork comprising a star network centered around an access node whichmultiplexes traffic from one or a plurality of optical network units(ONU), said ONU and said access node being directly joined by at leastone optical fiber; said remote nodes amplifying optical wavelengthdivision multiplexing signals which are transmitted on an optical fibercomprising the higher level network which said remote nodes belong to,branching the signals to an optical fiber comprising the lower levelnetwork, and coupling optical wavelength division multiplexing signals,input from an optical fiber comprising the lower level network, tooptical wavelength division multiplexing signals transmitted on anoptical fiber comprising said higher level network, and amplifying thecoupled signals; said access node amplifying the optical wavelengthdivision multiplexing signals transmitted from said optical fibers whichcomprise the higher level network which said access node is connectedto, selecting optical signals having wavelengths which correspond tosaid ONU, and outputting the selected signals to said ONU; multiplexingsaid optical signals transmitted from said ONU, dividing the multiplexedsignals in a plurality of directions, amplifying the divided signals,and transmitting the amplified signals to an optical fiber comprising ahigher level network which said access node is connected to; and thecenter node belonging to said highest level network and said ONUestablishing a direct communication path by using lights of differentwavelengths, the optical signals being amplified, branched, and routedat said remote nodes and said access node provided therebetween.
 2. Theoptical wavelength division multiplexing network as described in claim 1, wherein the highest level network and a network immediately therebelowcomprise a double ring, a ring network immediately therebelow comprisingone or a plurality of nodes, and a ring network therebelow comprisingaccess nodes.
 3. The optical wavelength division multiplexing network asdescribed in claim 2 , the access nodes where the optical signals fromsaid ONU are multiplexed transmitting and receiving optical wavelengthdivision multiplexing signals to/from the higher level nodes by using anoptical amplifier, an optical switch, and an opticalmultiplexer/de-multiplexer; communication between said center nodebelonging to said highest level network and said ONU being carried outby using optical amplifiers and passive optical components at a remotenode provided higher level than said access nodes.
 4. The opticalwavelength division multiplexing network as described in claim 3 , saidpassive optical components at the remote node, which is provided higherlevel than said access nodes which multiplex optical signals from saidONU, comprising optical couplers.
 5. The optical wavelength divisionmultiplexing network as described in claim 3 , said passive opticalcomponents at the remote node, which is provided higher level than saidaccess nodes which multiplex optical signals from said ONU, comprisingoptical circulators.
 6. An optical wavelength division multiplexingnetwork having a structure comprising at least two layers, a highestlevel network being a ring network which comprises at least one centernode and two or more remote nodes which are joined by at least twooptical fibers; a lowest level network comprising a star networkcentered around an access node which multiplexes traffic from one or aplurality of optical network units (ONU), said ONU and said access nodebeing directly joined by at least one optical fiber; an immediatelyhigher level network of said lowest level network being a ring networkcomprising at least one said access node connected by at least twofibers, traffic from said access nodes being multiplexed at a centernode in the ring network which said access node belongs to, andconnected by said center node to an even higher level network; saidremote node amplifying and branching optical wavelength divisionmultiplexing signals which are transmitted on an optical fibercomprising the higher level network which said remote node belongs to,de-multiplexing and receiving only optical signals at wavelengthscorresponding to said ONU, electrically processing said optical signals,and transmitting the processed signals at a predetermined wavelength tooptical fibers comprising a lower level network; de-multiplexing andreceiving only optical signals among the optical wavelength divisionmultiplexing signals, input along the optical fibers comprising thelower level network, which are at wavelengths corresponding to said ONU,electrically processing said optical signals, converting the processedsignals to optical signals at wavelengths which were allocatedbeforehand, and coupling the converted signals to optical wavelengthdivision multiplexing signals transmitted on optical fibers comprisingsaid higher level network; said access node provided between said remotenode and said ONU amplifying the optical wavelength divisionmultiplexing signals which are transmitted on the optical fiberscomprising the higher level network which the access node is connectedto, selecting optical signals which correspond to said ONU andoutputting the selected signals thereto; and multiplexing the opticalsignals from said ONU, dividing the multiplexed signal in a plurality ofdirections, amplifying the divided signals, and transmitting theamplified signals on optical fibers comprising the higher level networkwhich said access node is connected to; and optical signals havingdifferent wavelengths being transmitted between said ONU and the remotenode in the higher level network, which is the center node in the ringnetwork comprising said access node, said access node provided betweensaid remote node and said ONU amplifying and routing the opticalsignals.
 7. The optical wavelength division multiplexing network asdescribed in claim 6 , wherein the highest level network and the networkimmediately therebelow comprise a double ring, and a ring networktherebelow comprising access nodes.
 8. The optical wavelength divisionmultiplexing network as described in claim 7 , the access node, whereoptical signals from said ONU are multiplexed, transmitting andreceiving optical wavelength division multiplexing signals to/from theremote node, where traffic from said access node is multiplexed, byusing an optical amplifier, an optical switch, and an opticalmultiplexer/de-multiplexer; said remote node equalizing, identifying,and reproducing optical wavelength division multiplexing signals at eachwavelength from nodes belonging to said access nodes or higher levelnetworks, converting the signals to optical signals at a predeterminedwavelength and transmitting the converted signals to the higher levelnodes or said access nodes; passive optical components being used forextracting optical wavelength division multiplexing signals from saidhigher level network and transmitting optical wavelength divisionmultiplexing signals to said higher level network.
 9. The opticalwavelength division multiplexing network as described in claim 8 , saidpassive optical components at the remote node, which multiplexes trafficfrom said access nodes, uses optical couplers.
 10. The opticalwavelength division multiplexing network as described in claim 8 , saidpassive optical components at the remote node, which multiplexes trafficfrom said access nodes, uses optical circulators.
 11. The opticalwavelength division multiplexing network as described in claim 2 ,wherein, at a node belonging to both the lower level ring networkcomprising said access nodes and the ring network thereabove, both endsof at least two looped optical fibers, which join the access nodesbelonging to said lower level ring network, are open; and said accessnodes and said remote nodes do not comprise opticalmultiplexer/de-multiplexers having wavelength selectability, said ONUthemselves having a wavelength de-multiplexing function.
 12. The opticalwavelength division multiplexing network as described in claim 11 , allthe optical wavelength division multiplexing signals, transmitted on atleast two optical fibers which are used in said ring networks, arebi-directional, bi-directional amplifiers being used in said remotenodes and said access nodes.
 13. An optical wavelength divisionmultiplexing network comprising at least three layers, a highest levelnetwork being a ring network comprising at least one center node and twoor more remote nodes which are joined by at least four optical fibers;an intermediate level network being a ring network having a nodebelonging to the higher level network as a center node thereof, accessnodes belonging to said ring network being joined by at least fouroptical fibers; a lowest level network comprising a star networkcentered around an access node which multiplexes traffic from opticalnetwork units (ONU), said ONU and said access node being directly joinedby at least one optical fiber; said remote node amplifying opticalwavelength division multiplexing signals transmitted on said opticalfibers comprising a higher level node which the remote node belongs to,branching the signals to optical fibers comprising a lower levelnetwork, and coupling optical wavelength division multiplexing signalswhich are input from optical fibers comprising the lower level networkto optical wavelength division multiplexing signals transmitted onoptical fibers comprising said higher level network, thereby amplifyingthe coupled signals; said access node amplifying optical wavelengthdivision multiplexing signals transmitted on optical fibers comprising ahigher level network, which said access node belongs to, branching theamplified signals to a lower level network for outputting the branchedsignals to said ONU; multiplexing optical signals transmitted from saidONU, dividing the multiplexed signals in a plurality of directions,coupling the divided signal to optical wavelength division multiplexingsignals transmitted on optical fibers comprising a higher level networkwhich said access node is connected to, and amplifying the coupledsignals; and the center node belonging to said highest level network andsaid ONU establishing a direct communication path by using lights ofdifferent wavelengths, the optical signals being amplified, branched, orrouted, at said remote nodes and said access nodes providedtherebetween.
 14. The optical wavelength division multiplexing networkas described in claim 13 , wherein, at a node belonging to saidintermediate level ring network, both ends of at least four loopedoptical fibers, which join the access nodes belonging to said lowerlevel ring network, are open; and said access nodes and said remotenodes do not comprise optical multiplexer/de-multiplexers havingwavelength selectability, said ONU themselves having a wavelengthde-multiplexing function.
 15. An optical wavelength divisionmultiplexing network comprising at least three layers, a highest levelnetwork being a ring network comprising at least one center node and twoor more remote nodes which are joined by at least two optical fibers; anintermediate level network being a ring network having a node belongingto the higher level network as a center node thereof, access nodesbelonging to said ring network being joined by at least four opticalfibers; a lowest level network comprising a star network centered aroundan access node which multiplexes traffic from optical network units(ONU), said ONU and said access node being directly joined by at leastone optical fiber; said remote nodes amplifying optical wavelengthdivision multiplexing signals transmitted on said optical fiberscomprising a higher level network which said remote nodes belong to,branching the signals to optical fibers comprising a lower levelnetwork, and coupling optical wavelength division multiplexing signalswhich are input from optical fibers comprising the lower level networkto optical wavelength division multiplexing signals transmitted onoptical fibers comprising said higher level network, and amplifying thecoupled signals; said access node amplifying optical wavelength divisionmultiplexing signals transmitted on optical fibers comprising a higherlevel network, which said access node belongs to, branching them to alower level network for outputting the branched signals to said ONU;multiplexing optical signals transmitted from said ONU, dividing them ina plurality of directions, coupling the divided signals to opticalwavelength division multiplexing signals transmitted on optical fiberscomprising a higher level network which said access node is connectedto, and amplifying the coupled signals; and the center node belonging tosaid highest level network and said ONU establishing a directcommunication path by using lights of different wavelengths, the opticalsignals being only amplified, branched, or routed, at said remote nodesand said access node provided therebetween.
 16. The optical wavelengthdivision multiplexing network as described in claim 15 , wherein, at anode belonging to said intermediate level ring network, one end of atleast four looped optical fibers, which join the access nodes belongingto said lower level ring network, is open; and said access nodes andsaid remote nodes do not comprise optical multiplexer/de-multiplexershaving wavelength selectability, said ONU themselves having a wavelengthde-multiplexing function.
 17. An optical wavelength divisionmultiplexing network having a structure comprising at least two layers,a highest level network comprising a ring network having at least onecenter node and two or more remote nodes, which are joined by at leastfour optical fibers; intermediate level networks excepting the lowestlevel network comprising a ring network having a node belonging to thehigher level network as a center node, and at least one node belongingto the intermediate level ring networks being joined by at least fouroptical fibers; the lowest level network comprising a star networkcentered around an access node belonging to the ring network which isprovided immediately thereabove, said access node being joined to atleast one optical network unit (ONU) by at least two optical fibers;said remote nodes amplifying optical wavelength division multiplexingsignals transmitted on said optical fibers comprising a higher levelnode which said remote nodes belong to, branching the signals to opticalfibers comprising a lower level network; and coupling optical wavelengthdivision multiplexing signals which are input from optical fiberscomprising the lower level network to optical wavelength divisionmultiplexing signals transmitted on optical fibers comprising saidhigher level network; said access node amplifying optical wavelengthdivision multiplexing signals transmitted on optical fibers comprising ahigher level network which said access node is connected to, branchingthe amplified signals to a lower level network, amplifying the dividedsignals, and outputting the amplified signals to said ONU; multiplexingand amplifying optical signals transmitted from said ONU, dividing theamplified signals in a plurality of directions, coupling the dividedsignals to optical wavelength division multiplexing signals transmittedon optical fibers comprising a higher level network which said accessnode is connected to, and amplifying the coupled signals; and the centernode belonging to said highest level network transmitting data by usingdifferent wavelengths allocated to said ONU, said ONU transmitting thedata to said center node by using optical signals having the samewavelengths as the allocated wavelengths; and said access nodes and saidremote nodes provided between said center node and said ONU onlyamplifying and dividing, or routing, the optical signals.
 18. Thethree-layered optical wavelength division multiplexing network asdescribed in claim 17 , wherein the highest level network and thenetwork therebelow comprise a double ring structure, and the nodesbelonging to the lower level ring network comprise access nodes.
 19. Theoptical wavelength division multiplexing network as described in claim18 , one end of said lower level ring network are open at the remotenode, which is the center node in said lower level ring network,belonging to said higher level ring network.
 20. The optical wavelengthdivision multiplexing network as described in claim 19 , said accessnode separating the optical wavelength division multiplexing signals ateach wavelength, and distributing optical signals of differentwavelengths to each of said ONU.
 21. The optical wavelength divisionmultiplexing network as described in claim 19 , said access nodedistributing optical signals to said ONU by using star couplers, saidONU having a wavelength selection function.
 22. The optical wavelengthdivision multiplexing network as described in one of claims 1, 6, 13,15, and 17, wherein communication between said ONU and said access nodesis doubled by using radio communications.
 23. The optical wavelengthdivision multiplexing network as described in one of claims 1, 6, 13,15, and 17, said optical multiplexer/de-multiplexer provided at saidaccess node being provided at a remote terminal instead.
 24. The opticalwavelength division multiplexing network as described in claim 23 ,wherein communication between said ONU and said access nodes is doubledby using radio communications.
 25. A node apparatus in an opticalnetwork comprising at least two layers, the node apparatus, which isconnected to a highest level network and becomes the final multiplexingdestination of traffic, establishing a direct communication path tooptical network units (ONU) and transmitting data by means of opticalsignals at wavelengths allocated to said ONU, said ONU transmitting databy means of optical signals at wavelengths which are different from saidwavelength to the node apparatus which becomes the final multiplexingdestination of traffic, and the node apparatus connecting to a lowestlevel network having wavelength selectability; said node apparatus whichbecomes the final multiplexing destination of traffic comprising: aplurality of optical de-multiplexers which de-multiplex opticalwavelength division multiplexing signals, input from optical fiberscomprising said highest level network, to optical signals at eachwavelength; a plurality of optical receivers which convert the opticalsignals which have been de-multiplexed by said optical de-multiplexersto electrical signals; a plurality of selectors which selectively outputeither of the outputs from said plurality of optical receivers; a signaltermination section which performs predetermined electrical processingto the electrical signals which have been selected by said selectors,and outputs a plurality of groups of electrical signals; a plurality ofoptical senders which convert the electrical signals output from thesignal termination section to a plurality of optical signals havingdifferent wavelengths; and a plurality of optical multiplexers whichmultiplex the optical signals output from said optical senders, andoutput the multiplexed signals to optical fibers comprising said highestlevel network.
 26. A node apparatus in an optical network comprising atleast two layers, the node apparatus, which is connected to a highestlevel network and becomes the final multiplexing destination of traffic,establishing a direct communication path to optical network units (ONU)and transmitting data by means of optical signals at wavelengthsallocated to said ONU, said ONU transmitting data by means of opticalsignals at wavelengths which are different from said wavelength to thenode apparatus which becomes the final multiplexing destination oftraffic, and the node apparatus connecting to a lowest level networkhaving wavelength selectability; said node apparatus which becomes thefinal multiplexing destination of traffic comprising: a plurality ofoptical de-multiplexers which de-multiplex optical wavelength divisionmultiplexing signals, input from optical fibers comprising said highestlevel network, to optical signals at each wavelength; a plurality ofoptical switches which select one of the optical signal which have beende-multiplexed by said optical de-multiplexers; a plurality of opticalreceivers which convert the optical signals which have been selected bysaid optical switches to electrical signals; a signal terminationsection which performs predetermined electrical processing to theelectrical signals which have been output from said optical receivers,and outputs a plurality of groups of electrical signals; a plurality ofoptical senders which convert the electrical signals output from thesignal termination section to a plurality of optical signals havingdifferent wavelengths; and a plurality of optical multiplexers whichmultiplex the optical signals output from said optical senders, andoutput the multiplexed signals to optical fibers comprising said highestlevel network.
 27. A node apparatus in an optical network comprising atleast two layers, the node apparatus, which is connected to a highestlevel network and becomes the final multiplexing destination of traffic,establishing a direct communication path to optical network units (ONU)and transmitting data by means of optical signals at wavelengthsallocated to said ONU, said ONU transmitting data by means of opticalsignals at wavelengths which are different from said wavelength to thenode apparatus which becomes the final multiplexing destination oftraffic, and the node apparatus connecting to a lowest level networkhaving wavelength selectability; said node apparatus which becomes thefinal multiplexing destination of traffic comprising: a plurality ofoptical de-multiplexers which de-multiplex optical wavelength divisionmultiplexing signals, input from optical fibers comprising said highestlevel network, to a plurality of optical signals at each wavelength; aplurality of optical switches which select one of the plurality ofoptical signals which have been de-multiplexed by said opticalde-multiplexers; a plurality of optical receivers which convert theoptical signals which have been selected by said optical switches toelectrical signals; a signal termination section which performspredetermined electrical processing to the electrical signals which havebeen output from said optical receivers, and outputs a plurality ofgroups of electrical signals; a plurality of optical senders whichconvert the plurality of electrical signals output from the signaltermination section to a plurality of optical signals having differentwavelengths; a plurality of optical dividers which divide the opticalsignals output from said optical senders in a plurality of directions;and a plurality of optical multiplexers which multiplex the plurality ofoptical signals output from said optical dividers, and output themultiplexed signals to optical fibers comprising said highest levelnetwork.
 28. A node apparatus in an optical network comprising at leasttwo layers, the node apparatus, which is connected to a highest levelnetwork and becomes the final multiplexing destination of traffic,establishing a direct communication path to optical network units (ONU)and transmitting data by means of optical signals at wavelengthsallocated to said ONU, said ONU transmitting data by means of opticalsignals at wavelengths which are different from said wavelength to thenode apparatus which becomes the final multiplexing destination oftraffic, and the node apparatus connecting to a lowest level networkhaving wavelength selectability; the node apparatus being connected tonetworks other than said lowest level network, and comprising: passiveoptical components which branch optical signals transmitted on anoptical fiber comprising a higher level network to an optical fibercomprising a lower level network, and couple optical signals input froman optical fiber comprising said lower level network to optical signalstransmitted on an optical fiber comprising said higher level network;and optical amplifiers which amplify the optical signals input to saidpassive optical components and the optical signals output from saidpassive optical components.
 29. A node apparatus in an optical networkcomprising at least two layers, the node apparatus, which is connectedto a highest level network and becomes the final multiplexingdestination of traffic, establishing a direct communication path tooptical network units (ONU) and transmitting data by means of opticalsignals at wavelengths allocated to said ONU, said ONU transmitting databy means of optical signals at wavelengths which are different from saidwavelength to the node apparatus which becomes the final multiplexingdestination of traffic, the node apparatus being connected to saidlowest level network, and comprising: an optical switch which selectsone of the optical signals which are input from optical fiberscomprising a higher level network; a first optical amplifier whichamplifies, among the optical signals which are input from the opticalfibers comprising said higher level network, at least the optical signalselected by said optical switch; an optical multiplexer/de-multiplexerwhich, based on the optical signal selected by said optical switch,selects an optical signal having a wavelength which corresponds to saidONU, outputs the selected signal to said ONU, and multiplexes theoptical signals transmitted from said ONU; an optical divider whichdivides the optical signal, multiplexed by said opticalmultiplexer/de-multiplexer, into a plurality of directions, andtransmits the divided signals to the optical fibers comprising saidhigher level network; and a second optical amplifier which amplifies theoptical signals which are transmitted to the optical fibers comprisingsaid higher level network.
 30. A node apparatus which is connected to ahighest level network and becomes the final multiplexing destination oftraffic in an optical network comprising at least two layers, the nodeapparatus connected to a network immediately above a lowest levelnetwork establishing a direct communication path with optical networkunits (ONU) and transmitting data by using optical signals atwavelengths allocated to each of said ONU, and said ONU transmitting thedata to the node apparatus connected to a network provided higher levelthan said lowest level network by optical signals at wavelengths whichare different to the wavelengths, said node apparatus which is connectedto said highest level network comprising: a plurality of opticalde-multiplexers which de-multiplex optical wavelength divisionmultiplexing signals, input from optical fibers comprising said highestlevel network, to optical signals at each wavelength; a plurality ofoptical receivers which convert the optical signals which have beende-multiplexed by said optical de-multiplexers to electrical signals; aplurality of selectors which selectively output either of the outputsfrom said plurality of optical receivers; a signal termination sectionwhich performs predetermined electrical processing to the electricalsignals which have been selected by said selectors, and outputs aplurality of groups of electrical signals; a plurality of opticalsenders which convert the electrical signals output from the signaltermination section to a plurality of optical signals having differentwavelengths; and a plurality of optical multiplexers which multiplex theoptical signals output from said optical senders, and output themultiplexed signals to optical fibers comprising said highest levelnetwork.
 31. A node apparatus which is connected to a highest levelnetwork and becomes the final multiplexing destination of traffic in anoptical network comprising at least two layers, the node apparatusconnected to a network immediately above a lowest level networkestablishing a direct communication path with optical network units(ONU) and transmitting data by using optical signals at wavelengthsallocated to each of said ONU, and said ONU transmitting the data to thenode apparatus connected to a network provided higher level than saidlowest level network by optical signals at wavelengths which aredifferent to the wavelengths, said node apparatus which is connected tosaid highest level network comprising: a plurality of opticalde-multiplexers which de-multiplex optical wavelength divisionmultiplexing signals, input from optical fibers comprising said highestlevel network, to optical signals at each wavelength; a plurality ofoptical switches which select one of the optical signal which have beende-multiplexed by said optical de-multiplexers; a plurality of opticalreceivers which convert the optical signals which have been selected bysaid optical switches to electrical signals; a signal terminationsection which performs predetermined electrical processing to theelectrical signals which have been output from said optical receivers,and outputs a plurality of groups of electrical signals; a plurality ofoptical senders which convert the electrical signals output from thesignal termination section to a plurality of optical signals havingdifferent wavelengths; and a plurality of optical multiplexers whichmultiplex the optical signals output from said optical senders, andoutput the multiplexed signals to optical fibers comprising said highestlevel network.
 32. A node apparatus which is connected to a highestlevel network and becomes the final multiplexing destination of trafficin an optical network comprising at least two layers, the node apparatusconnected to a network immediately above a lowest level networkestablishing a direct communication path with optical network units(ONU) and transmitting data by using optical signals at wavelengthsallocated to each of said ONU, and said ONU transmitting the data to thenode apparatus connected to a network provided higher level than saidlowest level network by optical signals at wavelengths which aredifferent to the wavelengths, said node apparatus which is connected tosaid highest level network comprising: a plurality of opticalde-multiplexers which de-multiplex optical wavelength divisionmultiplexing signals, input from optical fibers comprising said highestlevel network, to a plurality of optical signals at each wavelength; aplurality of optical switches which select one of the plurality ofoptical signals which have been de-multiplexed by said opticalde-multiplexers; a plurality of optical receivers which convert theoptical signals which have been selected by said optical switches toelectrical signals; a signal termination section which performspredetermined electrical processing to the electrical signals which havebeen output from said optical receivers, and outputs a plurality ofgroups of electrical signals; a plurality of optical senders whichconvert the plurality of electrical signals output from the signaltermination section to a plurality of optical signals having differentwavelengths; a plurality of optical dividers which divide the opticalsignals output from said optical senders in a plurality of directions;and a plurality of optical multiplexers which multiplex the plurality ofoptical signals output from said optical dividers, and output themultiplexed signals to optical fibers comprising said highest levelnetwork.
 33. A node apparatus in an optical network comprising at leasttwo layers, the node apparatus connected to a network immediately abovea lowest level network establishing a direct communication path withoptical network units (ONU) and transmitting data by using opticalsignals at wavelengths allocated to each of said ONU, and said ONUtransmitting the data to the node apparatus connected to a networkprovided higher level than said lowest level network by optical signalsat wavelengths which are different to the wavelengths, the nodeapparatus being connected to a network which is provided higher levelthan said lowest level network, and comprising: passive opticalcomponents which branch optical signals transmitted on optical fiberscomprising the higher level network, and couple input optical signals tooptical signals transmitted on optical fibers comprising said higherlevel network; optical amplifiers which amplify the optical signalsinput to said passive optical components and the optical signals outputfrom said passive optical components; and an equipment for signaltermination which de-multiplexes only the optical signals among thosedivided by said passive optical components at wavelengths correspondingto said ONU, receives and electrically processes the optical signals ateach wavelength, and transmits the processed signals at a predeterminedwavelength, and in addition, de-multiplexes only the optical signalsamong those input along the optical fibers comprising a lower levelnetwork which are at wavelengths corresponding to said ONU, receives andelectrically processes the optical signals at each wavelength, convertsthe processed signals to optical signals at a wavelength allocatedbeforehand, and transmits the converted signals to said passive opticalcomponents.
 34. A node apparatus in an optical network comprising atleast two layers, the node apparatus connected to a network immediatelyabove a lowest level network establishing a direct communication pathwith optical network units (ONU) and transmitting data by using opticalsignals at wavelengths allocated to each of said ONU, and said ONUtransmitting the data to the node apparatus connected to a networkprovided higher level than said lowest level network by optical signalsat wavelengths which are different to the wavelengths, the nodeapparatus being connected to said lowest level network, and comprising:an optical switch which selects one of the optical signals which areinput from optical fibers comprising a higher level network; a firstoptical amplifier which amplifies, among the optical signals which areinput from the optical fibers comprising said higher level network, atleast the optical signal selected by said optical switch; an opticalmultiplexer/de-multiplexer which, based on the optical signal selectedby said optical switch, selects an optical signal having a wavelengthwhich corresponds to said ONU, outputs the selected signal to said ONU,and multiplexes the optical signals transmitted from said ONU; anoptical divider which divides the optical signal, multiplexed by saidoptical multiplexer/de-multiplexer, into a plurality of directions, andtransmits the divided signals to the optical fibers comprising saidhigher level network; and a second optical amplifier which amplifies theoptical signals which are transmitted to the optical fibers comprisingsaid higher level network.
 35. A node apparatus in an optical networkcomprising three layers, the node apparatus, which is connected to ahighest level network and becomes the final multiplexing destination oftraffic, establishing a direct communication path to optical networkunits (ONU) and transmitting data by using optical signals atwavelengths allocated to said ONU, said ONU transmitting data by usingoptical signals at wavelengths which are different from said wavelengthsto said node apparatus which becomes the final multiplexing destinationof traffic, nodes apparatuses other than said node apparatus whichbecomes the final multiplexing destination of traffic not having theability to select wavelengths, and said ONU having a function forde-multiplexing wavelengths, said node apparatus which becomes the finalmultiplexing destination of traffic comprising: a plurality of opticalde-multiplexers which de-multiplex optical wavelength divisionmultiplexing signals, input from optical fibers comprising said highestlevel network, to optical signals at each wavelength; a plurality ofoptical receivers which convert the optical signals which have beende-multiplexed by said optical de-multiplexers to electrical signals; aplurality of selectors which selectively output either of the outputsfrom said plurality of optical receivers; a signal termination sectionwhich performs predetermined electrical processing to the electricalsignals which have been selected by said selectors, and outputs aplurality of groups of electrical signals; a plurality of opticalsenders which convert the electrical signals output from the signaltermination section to a plurality of optical signals having differentwavelengths; and a plurality of optical multiplexers which multiplex theoptical signals output from said optical senders, and output themultiplexed signals to optical fibers comprising said highest levelnetwork.
 36. A node apparatus in an optical network comprising threelayers, the node apparatus, which is connected to a highest levelnetwork and becomes the final multiplexing destination of traffic,establishing a direct communication path to optical network units (ONU)and transmitting data by using optical signals at wavelengths allocatedto said ONU, said ONU transmitting data by using optical signals atwavelengths which are different from said wavelengths to said nodeapparatus which becomes the final multiplexing destination of traffic,nodes apparatuses other than said node apparatus which becomes the finalmultiplexing destination of traffic not having the ability to selectwavelengths, and said ONU having a function for de-multiplexingwavelengths, the node apparatus being connected to networks other thansaid lowest level network, and comprising: passive optical componentswhich branch optical signals transmitted on optical fibers comprising ahigher level network to optical fibers comprising a lower level network,and in addition, couple optical signals input from optical fiberscomprising said lower level network to optical signals transmitted onoptical fibers comprising said higher level network; and opticalamplifiers which amplify optical signals which are input to, and outputfrom, said passive optical components; wherein both ends of the loop ofoptical fibers comprising said lower level network are opened by usingoptical terminators.
 37. A node apparatus in an optical networkcomprising three layers, the node apparatus, which is connected to ahighest level network and becomes the final multiplexing destination oftraffic, establishing a direct communication path to optical networkunits (ONU) and transmitting data by using optical signals atwavelengths allocated to said ONU, said ONU transmitting data by usingoptical signals at wavelengths which are different from said wavelengthsto said node apparatus which becomes the final multiplexing destinationof traffic, nodes apparatuses other than said node apparatus whichbecomes the final multiplexing destination of traffic not having theability to select wavelengths, and said ONU having a function forde-multiplexing wavelengths, the node apparatus being connected to alowest level network, and comprising: first passive optical componentswhich branch optical signals transmitted on optical fibers comprising ahigher level network to a lower level network; an optical switch whichselects one of the optical signals which have been branched by saidfirst passive optical components; an optical multiplexer/de-multiplexerwhich transmits the optical signals selected by said optical switchtoward said ONU, and multiplexes the optical signals transmitted fromsaid ONU; an optical divider which divides the optical signalsmultiplexed by said optical multiplexer/de-multiplexer in a plurality ofdirections; second passive optical components which couple opticalsignals divided by said optical divider to optical signals transmittedon optical fibers comprising said higher level network; and opticalamplifiers which amplify the optical signals which are input to andoutput from said first and second passive optical components.
 38. A nodeapparatus in an optical network comprising three layers, the nodeapparatus, which is connected to a highest level network and becomes thefinal multiplexing destination of traffic, establishing a directcommunication path to optical network units (ONU) and transmitting databy using optical signals at wavelengths allocated to said ONU, said ONUtransmitting data by using optical signals at wavelengths which aredifferent from said wavelengths to said node apparatus which becomes thefinal multiplexing destination of traffic, nodes apparatuses other thansaid node apparatus which becomes the final multiplexing destination oftraffic not having the ability to select wavelengths, and said ONUhaving a function for de-multiplexing wavelengths, the node apparatusbeing connected to networks other than a lowest level network, andcomprising: passive optical components which branch optical signalstransmitted on optical fibers comprising a higher level network tooptical fibers comprising a lower level network, and in addition, coupleoptical signals input from optical fibers comprising said lower levelnetwork to optical signals transmitted on optical fibers comprising saidhigher level network; and optical amplifiers which amplify opticalsignals transmitted on the optical fibers comprising said higher levelnetwork; wherein one end of the loop of optical fibers comprising saidlower level network is opened by using optical terminators.
 39. A nodeapparatus in an optical network comprising at least two layers, the nodeapparatus, which is connected to a highest level network and becomes thefinal multiplexing destination of traffic, establishing a directcommunication path to optical network units (ONU) and transmitting databy using optical signals at wavelengths allocated to said ONU, said ONUtransmitting data by using optical signals at same wavelengths as saidwavelengths to said node apparatus which becomes the final multiplexingdestination of traffic, said node apparatus which becomes the finalmultiplexing destination of traffic comprising: a plurality of opticalde-multiplexers which de-multiplex optical wavelength divisionmultiplexing signals, input from optical fibers comprising said highestlevel network, to optical signals at each wavelength; a plurality ofoptical receivers which convert the optical signals which have beende-multiplexed by said optical de-multiplexers to electrical signals; aplurality of selectors which selectively output either of the outputsfrom said plurality of optical receivers; a signal termination sectionwhich performs predetermined electrical processing to the electricalsignals which have been selected by said selectors, and outputs aplurality of groups of electrical signals; a plurality of opticalsenders which convert the electrical signals output from the signaltermination section to a plurality of optical signals having differentwavelengths; and a plurality of optical multiplexers which multiplex theoptical signals output from said optical senders, and output themultiplexed signals to optical fibers comprising said highest levelnetwork.
 40. A node apparatus in an optical network comprising at leasttwo layers, the node apparatus, which is connected to a highest levelnetwork and becomes the final multiplexing destination of traffic,establishing a direct communication path to optical network units (ONU)and transmitting data by using optical signals at wavelengths allocatedto said ONU, said ONU transmitting data by using optical signals at samewavelengths as said wavelengths to said node apparatus which becomes thefinal multiplexing destination of traffic, the node apparatus beingconnected to networks other than said lowest level network, andcomprising: first passive optical components which branch opticalsignals transmitted on optical fibers comprising a higher level networkto optical fibers comprising a lower level network; second passiveoptical components which couple optical signals input from opticalfibers comprising said lower level network to optical signalstransmitted on optical fibers comprising said higher level network; andoptical amplifiers which amplify optical signals which are input to, andoutput from, said first and second passive optical components; whereinboth ends of the loop of optical fibers comprising said lower levelnetwork are opened by using optical terminators.
 41. A node apparatus inan optical network comprising at least two layers, the node apparatus,which is connected to a highest level network and becomes the finalmultiplexing destination of traffic, establishing a direct communicationpath to optical network units (ONU) and transmitting data by usingoptical signals at wavelengths allocated to said ONU, said ONUtransmitting data by using optical signals at same wavelengths as saidwavelengths to said node apparatus which becomes the final multiplexingdestination of traffic, the node apparatus being connected to saidlowest level network, and comprising: first passive optical componentswhich branch optical signals transmitted on optical fibers comprising ahigher level network to a lower level network; an optical switch whichselects one of the optical signals which have been branched by saidfirst passive optical components; a first optical amplifier whichamplifies, among the optical signals which have been branched by saidfirst passive optical components, at least the optical signal selectedby said optical switch; a second passive optical component whichdistributes the optical signals amplified by said first opticalamplifier to said ONU, and multiplexes the optical signals transmittedfrom said ONU; a second optical amplifier which amplifies the opticalsignals multiplexed by said second passive optical component; an opticaldivider which divides the optical signal, amplified by said secondoptical amplifier, into a plurality of directions; a third passiveoptical component which couples the optical signals branched by saidoptical divider to an optical signal transmitted on optical fiberscomprising said higher level network; and a third optical amplifierwhich amplifies the optical signals which are transmitted on the opticalfibers comprising said higher level network.
 42. A system switchingmethod in an optical wavelength division multiplexing network having astructure comprising at least two layers, a highest level network beinga ring network which comprises at least one center node and two or moreremote nodes which are joined by at least two optical fibers; in thecase where the layered structure comprises three or more layers,excepting the lowest level network the intermediate level networkcomprising a ring having said node belonging to the highest levelnetwork as its center node, nodes belonging to said ring network beingjoined by at least two optical fibers; said lowest level networkcomprising a star network centered around an access node whichmultiplexes traffic from one or a plurality of optical network units(ONU), said ONU and said access node being directly joined by at leastone optical fiber; the center node belonging to said highest levelnetwork and said ONU establishing a direct communication path by usinglights of different wavelengths, the optical signals being amplified,branched, and routed at said remote nodes and said access node providedtherebetween; the system switching method comprising the followingsteps: when an optical fiber (working fiber), which is being used intransmitting a down signal from said center node to said ONU in saidhigher level network, becomes severed, an access node belonging to aremote node provided downstream than the severance point as seen fromsaid center node, switches from the working fiber side to an opticalfiber side (protection fiber) which is not presently in use, said downsignal being received after transmission along said protection fiber;when a working fiber for transmitting an up signal from said ONU to saidcenter node in said higher level network has become severed, for anaccess node belonging to remote node where the severance point on theworking fiber to said center node is located, said center node switchesfrom the working fiber to a protection fiber, and receives the up signalfrom said protection fiber; and when an optical cable in saidintermediate level network has become severed, an access node, among theaccess nodes connected to said intermediate level network, which isprovided downstream than the severance point for the optical signaltransmitted on the severed fiber switches from the working fiber to theprotection fiber and thereby receives said down signal; and at saidaccess node provided downstream, said center node switches from theworking fiber to the protection fiber and thereby receives said upsignal from said protection fiber.
 43. A system switching method in anoptical wavelength division multiplexing network comprising at leastthree layers, a highest level network being a ring network comprising atleast one center node and two or more remote nodes which are joined byat least four optical fibers; an intermediate level network being a ringnetwork having a node belonging to the higher level network as a centernode thereof, access nodes belonging to said ring network being joinedby at least four optical fibers; a lowest level network comprising astar network centered around an access node which multiplexes trafficfrom optical network units (ONU), said ONU and said access node beingdirectly joined by at least one optical fiber; the center node belongingto said highest level network and said ONU establishing a directcommunication path by using lights of different wavelengths, the opticalsignals being amplified, branched, or routed at said remote nodes andsaid access nodes provided therebetween; the system switching methodcomprising the following steps: when an optical fiber (working fiber),which is being used in transmitting a down signal from said center nodeto said ONU in said higher level network, becomes severed, an accessnode belonging to a remote node provided downstream than the severancepoint as seen from said center node, switches from the working fiberside to an optical fiber side (protection fiber) which is not presentlyin use, said down signal being received after transmission along saidprotection fiber; when a working fiber for transmitting an up signalfrom said ONU to said center node in said higher level network hasbecome severed, for an access node belonging to remote node where theseverance point on the working fiber to said center node is located,said center node switches from the working fiber to a protection fiberand receives the up signal from said protection fiber; and when anoptical cable in said intermediate level network has become severed, anaccess node, among the access nodes connected to said intermediate levelnetwork, which is provided downstream than the severance point for theoptical signal transmitted on the severed fiber switches from theworking fiber to the protection fiber and thereby receives said downsignal; and at said access node provided downstream, said center nodeswitches from the working fiber to the protection fiber and therebyreceives said up signal from said protection fiber.
 44. A systemswitching method in an optical wavelength division multiplexing networkcomprising at least three layers, a highest level network being a ringnetwork comprising at least one center node and two or more remote nodeswhich are joined by at least two optical fibers; an intermediate levelnetwork being a ring network having a node belonging to the higher levelnetwork as a center node thereof, access nodes belonging to said ringnetwork being joined by at least four optical fibers; a lowest levelnetwork comprising a star network centered around an access node whichmultiplexes traffic from optical network units (ONU), said ONU and saidaccess node being directly joined by at least one optical fiber; thecenter node belonging to said highest level network and said ONUestablishing a direct communication path by using lights of differentwavelengths, the optical signals being only amplified, branched, androuted at said remote nodes and said access node provided therebetween;the system switching method comprising the following steps: when anoptical fiber (working fiber), which is being used in transmitting adown signal from said center node to said ONU in said higher levelnetwork, becomes severed, an access node belonging to a remote nodeprovided downstream than the severance point as seen from said centernode, switches from the working fiber side to an optical fiber(protection fiber) which is not presently in use, said down signal beingreceived after transmission along said protection fiber; when a workingfiber for transmitting an up signal from said ONU to said center node insaid higher level network has become severed, for an access nodebelonging to remote node where the severance point on the working fiberto said center node is located, said center node switches from theworking fiber to a protection fiber, and receives the up signal fromsaid protection fiber; and when an optical cable in said intermediatelevel network has become severed, an access node, among the access nodesconnected to said intermediate level network, which is provideddownstream than the severance point for the optical signal transmittedon the severed fiber switches from the working fiber to the protectionfiber and thereby receives said down signal; and at said access nodeprovided downstream, said center node switches from the working fiber tothe protection fiber and thereby receives said up signal from saidprotection fiber.
 45. A system switching method in an optical wavelengthdivision multiplexing network having a structure comprising at least twolayers, a highest level network comprising a ring network having atleast one center node and two or more remote nodes, which are joined byat least four optical fibers; intermediate level networks excepting thelowest level network comprising a ring network having a node belongingto the higher level network as a center node, and at least one nodebelonging to the intermediate level ring networks being joined by atleast four optical fibers; the lowest level network comprising a starnetwork centered around an access node belonging to the ring networkwhich is provided immediately thereabove, said access node being joinedto at least one optical network unit (ONU) by at least two opticalfibers; the center node belonging to said highest level networktransmitting data by using different wavelengths allocated to said ONU,said ONU transmitting the data to said center node by using opticalsignals having the same wavelengths as the allocated wavelengths; andsaid access nodes and said remote nodes provided between said centernode and said ONU only amplifying and dividing, or routing, the opticalsignals, the system switching method comprising the following steps:when an optical fiber (working fiber), which is being used intransmitting a down signal from said center node to said ONU in saidhigher level network, becomes severed, an access node belonging to aremote node provided downstream than the severance point as seen fromsaid center node, switches from the working fiber side to an opticalfiber (protection fiber) which is not presently in use, said down signalbeing received after transmission along said protection fiber; when aworking fiber for transmitting an up signal from said ONU to said centernode in said higher level network has become severed, for an access nodebelonging to remote node where the severance point on the working fiberto said center node is located, said center node switches from theworking fiber to a protection fiber, and receives the up signal fromsaid protection fiber; and when an optical cable in said intermediatelevel network has become severed, an access node, among the access nodesconnected to said intermediate level network, which is provideddownstream than the severance point for the optical signal transmittedon the severed fiber switches from the working fiber to the protectionfiber and thereby receives said down signal; and at said access nodeprovided downstream, said center node switches from the working fiber tothe protection fiber and thereby receives said up signal from saidprotection fiber.